LOL--you guys are funny--All of the above listed items could be consolidated into one name---
1. New features----new features such as ammo making and six and others are just simply that--NEW FEATURES.
That is not an improvement.
Don't be silly, new features (especially new features that dramatically change the mechanics of the game for the better, like damage threshold or ammunition types) are most certainly an improvement.
There is a difference between, New Features, New Improvements, Factual improvements and Opinion Improvements.
What's the difference between a new feature and an improvement?
What new additions or new features are in Fallout New Vegas that could not be added as DLC to Fallout 3?
Ammunition types. Damage threshold. Weapon mods*. Faction mechanics/reputation.
Off the top of my head. Weapon mods is somewhat iffy because there was a mod that implemented them, but it required FOSE for most of its features,
and the code was pretty clearly a workaround rather than something actually organically implemented in the game.
Give me 3 things that could not be added as DLC and are REAL game improvements...not new features, something that has changed the gameplay so significantly that makes New Vegas improved over Fallout 3.
Damage Threshold. In Fallout 3, DR knocks a flat percentage off of every hit; up to about ~50% with high end power armor. Your weapon's stats don't really matter; there's no significant benefit or drawback to frontloaded damage versus DPS through rate of fire. Two weapons with equal DPS, even if one is an SMG with low DAM and high rate of fire and the other is a sniper rifle with high DAM but low ROF, will kill the target in the same time.
In New Vegas, DT automatically reduces all incoming damage by X; up to around 32ish for Remnants PA. What this means is that low damage but high DPS weapons (example: SMGs) are less effective versus armor and more effective versus unarmored targets. There is some bleedthrough in NV so that even an 8 DAM 9mm SMG will do some damage even if stopped by DT (around 2-3 IIRC), however, this change still makes combat more nuanced. It makes both high DAM and high DPS weapons have a place in your arsenal. That big beefy anti-material rifle is awesome for shooting through thickly-armored deathclaws, but not so good if you get swarmed by cazcadores. That gatling laser will utterly fry the swarm, but it's not powerful enough per shot to deal with deathclaws before they tear you apart.
Adding DT as a mechanic couldn't be done in a DLC or patch; it would require you to recode heavily and you'd have to completely rebalance the game.
Now, let's talk about ammunition types. This is similarly a huge improvement. They let you change the characteristics of your weapon on the fly; need to get through a pesky centurion's armor? Load up AP ammo. Dealing with a bunch of unarmored fiends and/or fire geckos? Hollow Point ammo is what the doctor ordered. By allowing you to tailor ammunition to the target, the game gives the players vastly increased options. Each encounter becomes more nuanced and interesting. Before, the only option you had was what weapon you use. Now you've got a choice of weapon (and more reason to choose a particularly weapon over another for any given situation), as well as what type of ammo to use. Do you choose JSP's damage and accuracy bonus, even though it doesn't negate 18 DT like AP ammo does? How about .50 BMG Match, even though it doesn't light your enemies on fire like .50 BMG Incendiary does?
Ammo changing couldn't be implemented in a DLC or patch because you'd have to do some pretty heavy codework (the mod to do the same required FOSE), and you'd also have to rebalance the whole game.
Weapon modding is similarly a big deal. In New Vegas, you can turn a laser rifle into a long ranged laser sniper rifle with the addition of a scope and a focusing lens. You can extend a shotgun's magazine, or even tighten its spread to let it be more useful at long range or for those ever-important headshots. And then there's the plasma rifle! Bar none, the big issue with plasma weapons in FO3 was how the bolts travel too slowly. Lo and behold, now there's a weapon mod to rectify that issue. The addition of silencers and more silenced weapons is big too - in FO3, the only silenced weapon was the silenced 10mm pistol. But by adding a silencer to a 12.7mm SMG or a sniper rifle, you open up tactical options for any sort of sneak-based character.
Finally, the reputation system is a huge blow for verisimilitude. In Fallout 3, you could murder a bunch of people in any given settlement, wait three days, then come back and find everyone treating you just fine. In New Vegas, murdering the inhabitants of, say, Cottonwood Cove means that the entire legion will hate and fear you. As you'd expect of someone who just murdered a town full of their friends and comrades. Further, the hit squads that spawn to kill you are a result of your actions and not your karma. In Fo3, if you gave enough water to beggars, Talon Company would show up to kill you. If you stole enough metal forks and worthless dinnerplates, the regulators would show up to kill you. In New Vegas, you only get hit squads sent after you if you do things against that faction; if you start indiscriminately murdering NCR troops or Legionnaires or working against either faction's interests. An altogether more intelligent and plausible mechanic than "lolz, you're a good person? Well, we're going to kill you because of that!!!"
Also, companion quests are a huge step forward. Those quests do a hell of a lot to humanize the companions and make them seem like real characters, rather than walking loot storage bins. The companion with the most development in FO3 is Fawkes. Yet even ED-E has more development then he does, due almost entirely to ED-E's companion quest. To say nothing of Boone, Raul, Veronica, or any of the other companions save possibly Rex. More developed characters in general is a big deal for an RPG.