Fallout New Vegas is built on top of the same engine as Fallout 3, but Obsidian has introduced or changed some key elements of Fallout 3. Some of these take the Fallout series back towards its roots, others expand on things that were only possible via mods in Fallout 3.

Damage Thresholds

One of the new concepts in Fallout New Vegas is "damage thresholds" or "DT". In Fallout 3, armor and clothing only had "damage resist" (DR) stats, which takes the incoming damage and reduces it by a set percentage. Between armor, permanent boosts from perks, and temporary boosts from chems, you could easily reach the max DR of 85% so that you only took 15% damage from any source. A hail of small bullets could still take you down if you managed to aggro a large enough group of enemies.

The concept of DT is also concerned with limiting incoming damage, but works on a threshold principle. Any individual source of damage that fails to do more then the DT amount will basically do no damage at all. There's still some confusion over how exactly DT works in New Vegas. Some say that if a strike would do 20 damage and you have 5 DT, you'd only take 15 damage total. If you get hit by 5 damage and have 10 DT you might take zero damage or some are saying that you'll still take 1 damage.

Regardless of the actual math, DT makes you and enemies that are wearing good armor pretty much immune to small arms fire. If you get struck and it was less then your DT, a small shield will flicker by your health bar. If you strike an enemy and DT makes your damage ineffective then a red shield will appear by your target's health bar. A good test of this is to save and then attack Victor (the robot) in the starting town of Goodsprings. Your measly 9mm gun will not dent him unless you change ammo.

(There were mods in Fallout 3 that introduced the idea of damage thresholds. They generally required FOSE to be installed because the basic FO3 engine did not provide the scripting functions needed.)

MMM Hunting and Looting

For those who used Mart's Mutant Mod (MMM), they will remember an optional portion of that mod called "Hunting & Looting". The idea being that NPCs would scoop up weapons and ammo from their fallen brethren to use against you or other NPCs. So if you kill the rocket-wielding bandit, the remaining bandits might pick it up and then use it. In the original Fallout 3, they would simply ignore such a useful weapon and continue to attack you with tire irons and rusty knives.

I've seen enemy NPCs in Fallout New Vegas run over and loot the bodies of fallen allies. I suspect that we're seeing the "looting" part of the MMM H&L mod in action, but I have not played enough yet to prove it.

Repair

Weapon condition still plays a factor in Fallout New Vegas. Just like in FO3, you need to keep your weapon in as good condition as possible. There's a "Repair" skill, but the process now works differently.

In FO3, your Repair skill (or the NPC merchant's skill) determined the maximum possible weapon quality that you could achieve, even if you had unlimited spares to use. In New Vegas, the repair skill is more about how efficient you are when you repair one item with another similar item. Early on, repairing a 9mm gun with another 9mm gun might result in +10% condition. But with higher repair skills, you might manage to get +20% condition when repairing the same pair of weapons. If you have a bunch of 9mm pistols, go ahead and repair at least one of them up to near 100%.

The bigger use for the Repair skill is now that it unlocks more difficult recipes at the Repair Bench objects scattered around the wastelands.

Ammo

Ammo is back and in hardcore mode, it has mass. The allowances for ammo weight are generous, but you won't be carrying 10,000+ rounds of ammo unless you are superman.

The bigger change is that there are now different types of ammo for each weapon. You might find ammo that improves the condition of your gun when you use it, or that is better at penetrating armor (armor piercing), or which does more damage against unarmored targets (hollow point). You can even create these different types of ammo at the ammo press objects.

Crafting

A new feature in New Vegas is the various crafting skills. I've discovered (3) basic crafting types and there's probably a 4th for chems? The tools used are the campfire, the repair bench and the ammo press. You'll be limited by either the Survival skill, the Repair skill or the Science skill. Higher skills in those areas will unlock more advanced recipes.

The campfire lets you turn scavenged items into things like food and water. The repair bench lets you create various items just like in Fallout 3, except that the recipes make more sense and the entire UI has been overhauled. The ammo press lets you create ammo by breaking down and reusing other ammo sources.

So if you see shell casings or spent power cartridges laying around, they're now worth picking up. Stash them somewhere in a central location and you might eventually be able to turn them into usable ammo.

Steam Integration

A lot of PC gamers hate Valve's Steam. Personally, I don't mind it, but I have 3.3GB of RAM installed and a quad-core CPU. Folks with only 1GB of RAM will probably have more of an issue because Steam does eat up 80-100MB of RAM at times. I like the easy purchase and download of games and not having to keep track of dozens of CDs (or inserting a CD to play a game).

However, the biggest annoyance is the use of [Shift-Tab] to bring up the Steam interface in-game. If you like to sneak around, you're generally crouched with the [Left-Control] key and probably trying to walk slowly [Left-Shift] as you sneak around. If you then decide to bring up your Pipboy with [Tab] while you have that [Left-Shift] key held down, you'll end up opening the in-game Steam UI.

Fortunately, you can remap the Steam UI key to something that is harder to hit by accident like [Ctrl-Tab]. Use the [Shift-Tab] to open up Steam, then look for "Settings" along the bottom of the screen.

Hardcore Mode

Another new feature in Fallout New Vegas is the "hardcore" mode. Before leaving Doc Mitchell's at the start, you will be prompted whether to play in hardcore mode. If you do so, you'll have to keep track of how much food / water / sleep you need in addition to tracking things like radiation and health. Crippled limbs will also not regenerate just by resting, you will need to use things like Doctor Bags or visit a NPC doctor to be fully healed. Hardcore mode rules in Fallout New Vegas are somewhat similar to those of various mods in Fallout 3 (like FWE or FO3 Wanderers Edition).

Note that in order to get the achievement - you must say "yes" when first prompted and then never turn that mode off for the rest of the game.

You can see your current condition by using the [F1] key and looking at the first tab (Status). In addition to the traditional "CND" (condition), "RAD" (radiation), and "EFF" (current effects) screens, there are now "H2O" (water), "FOD" (food) and "SLP" (sleep) meters.

Lower numbers are better, but you can't go below zero. As your meters get higher, you'll start to encounter negative effects once an individual meter rises above 200 points. Those effects can be temporarily losing a point of Endurance (END) or lower strength or agility or other negative impacts.

Some foods will increase your water meter while reducing your food meter, some only affect one or the other. Your character will die after about 2 weeks without water or 5 weeks without food or about 2 weeks without sleep.

Hardcore mode also makes ammo have weight. It's not a real big deal, but you will be limited to carrying only a few hundred rounds of ammunition instead of thousands of rounds (or tens of thousands) like you could in Fallout 3. All weight numbers are generally displayed to 2 decimal points (1/100th of a pound) instead of being rounded off to a single decimal point like was done in Fallout 3.

Another change between normal mode and hardcore mode is that Stimpaks and RadAway are no longer instant. They have been changed to work over time (5-10 seconds). This means that you'll want to use them sooner rather then later when damaged as a sudden burst of incoming damage might kill you before they take effect. In addition, using more then (4) stimpaks within a very short amount of time will afflict you with Stimpak Sickness that temporarily reduces your AGI and STR by one point.

Lastly, hardcore mode means that your companions can permanently die (unless you reload from a save). This is similar to Fallout 3, but in New Vegas they apparently changed it so that companions are "essential" so that they only fall unconscious if their hit points are reduced to zero. I'm basing that on hearsay from the Fallout 3 wiki and have not personally tested it yet.

Overall, hardcore mode doesn't seem to be that big of a deal. In Fallout 3, you only carried food/water for times when you wanted to keep the water beggars healthy for Karma or for when you didn't have easy access to weightless stimpaks. It definitely makes the various recipes you can use at campfires more useful and food/water becomes less of a "lesser stimpak" and more of a resource to be managed. The only major downside that I've found so far is that limb damage requires a (somewhat rare) doctor's kit or a visit to a doctor. So you'll need to play with a bit more finesse and avoid combat with enemies that can easily cripple you.

Iron Sights

The concept of iron sights in first-person shooters is that you look down the barrel and line up your target using the sights on the weapon model. It was a staple of games like the original Call of Duty and others but was initially missing from Fallout 3. Instead, in Fallout 3, you were given a graphical crosshair in the middle of the screen and the weapon model would sort of float off to the side, raising up a bit if you used the right-mouse button to aim but otherwise not obstructing your sight picture.

In New Vegas, you have the option to use iron sights. When you press the right-mouse button the crosshair vanishes to be replaced with the sights on the weapon that you're currently using. Generally regarded to be more immersive, iron sights aren't to everyone's taste and can be turned off.

No word yet whether it makes the weapons any more accurate. Using the right-mouse in Fallout 3 did improve accuracy, but you could still easily miss enemies at long range if your weapon skill was not high. I see no reason why F:NV would have changed that, even with the addition of iron sights. If you want accuracy, get your weapon skill up in conjunction with aimed shots using the right-mouse button.