They are just mods, though. Honestly, you're not necessarily missing out on all that much. I can't really think of any absolute must-have mods that I absolutely couldn't play Fallout 3 with. I've actually taken a number of them out after awhile, or simply forget that they're even loaded. The very best ones just do subtle things, and I can't say I've really come across a ton of additional content that feels like it would have belonged in a DLC or anything. (That's not a rag against the skill of the modders - I just think there's something to be said for working in a team on a singular vision with a creative director to make sure everything looks like it "belongs" together. Kind of the same reason that I don't generally download third-party content for my Sims games - they might be of high quality, but they often seem to stand out from the rest of the content, which I find jarring.)
I generally buy games like F3 on my computer, and the modding capability is a part of that reason; but I certainly wouldn't say it's worth it buy a whole computer specifically for that one purpose.
Back to topic: For NV, I'm kind of hoping to see some sort of arena deathsport thingy. Something that you could bet on (since it's in Vegas, after all,) and of course join in on.
I could not dis-agree more with this assessment, you clearly have not played MMM, FWE, WMK, FOOK, Foes Reworked, Phalanx, etc. If you had and REALLY played with them, you would never say this.
The big mod kits (several mentioned above) actually make Radical changes to how the game plays, the monsters you see, the weapons they use, the tactics they use, the loot you find, etc, etc. About the ONLY thing that is the same are the quests content/dialogue. But try walking anywhere in the wasteland at night and meet your first band of night ghouls! The weapon mod kits are Awesome, allowing you to build custom weapons and even make ammo. The changes to followers and NPC combat are also Very profound. An NPC that charges you (vanilla) versus one that ducts back and find good cover before laying down sniper fire on out is such a Radical change and improvement in the level of challenge, that it barely resbemles the vanilla game.
I strongly recommend that those who have not used the big mod kits before and have a PC, should try them at depth. They Really Do make the game alot more fun and challenging, and I couldn't imagine playing vanilla Fallout 3 anymore. There isn't anything wrong with nu_clear_day's opinion, but I wanted to make it very clear that this opinion is definitely not common in the mods forums, and people should try the big mods and make their own judgements before believing what is said here.
Also back on topic, this is one aspect of New Vegas that I am very hopeful about - the ability to mod the NV content when it comes out. If it were purely a Bethesda decision, I would be confident that we are going to have permission to mod with NV content. But this is Obsidian, and they may have different rules. We can hope though!

Miax