I am FAR more likely to pay good money for a high-quality expansion pack that's essentially a "Part 2" to the main game experience. I'm an older gamer, so I find the whole DLC thing kind of a "nickle-and-dime" affront to my intelligence, since when all is said and done, you're ultimately paying far more per "second of awesome" than you would in a much larger release. Not to mention that expansions tend to be coherent, and DLC is just, as another poster put it, a tack-on arcade game with little to do with anything. Plus, short DLC doesn't get my motor going -- "Oh boy, ANOTHER single quest line that doesn't really tie into the game. Joy." -- for this kind of stuff, I'm going to go to the modders who tend to try and tie in their stuff more neatly and aren't doing it for the money.
Please, PLEASE deliver an expansion.

And if you do, I've only got a few requests:
+ Make it pace well and really a part of the "main game" experience. I don't want to jump on a caravan and be magically transported to Antartica, where nothing in the main game effects it and it doesn't effect the main world, ala Shivering Isles. Throw me a bone here, please. Seamlessness.
+ PLEASE, for all that is good in the world, PLEASE... make Caesar's Legion more "worth it" to play. Right now, it's little more than a tribute to [censored] hack-and-slasher gameplay, and I expect so much more from you folks... You could have easily achieved the same thing in the "independent" game ending. If I wanted to run through the game without any real story and no side-quests and just gun down everything that moved, then I would have saved my game and then done just that for my jollies -- I don't need CL for it. It had so much potential for sixiness and underperformed -- I mean, the game shipped with 8 companions and not a one of them (save the dog/bot) are playable if you're in CL and 70% of the game's side stories are sunk in NCR goodness.
+ PLEASE do not push a "good vs. evil" storyline anymore. The game was marketed to me as one that would present me with morally ambiguous decisions, and that fell short. Except for the Bitter Springs side-story, the NCR is made out to be all puppies and rainbows and good will for all, when in 'reality' it's as big of a power-mongering machine and as full of nasty, racist/classist/fascist individuals as CL. If there's a NCR-leaning quest, give it an awesome CL alternative. One that doesn't involve "slip in and kill EVERYONE."