I'm going to preface that these answers will be for an X-COM D&D game only - I expect less RP and more death in this specific campaign than I would normally sign up for in a normal game. With the disclaimer out of the way:
Amount of Out-of-Combat Roleplaying - 3. Other than the squad, commanders and the occasional mysterious backer in a black monolith, I expect to not interact with anything other than people caught in the crossfire of the missions.
Complexity of the Plot – 3. The same level of complexity as the videogame is fine, but hopefully with different twists.
Linearity of the Plot – 3. I want to play a grunt that goes where ordered, kills what shoots back, and rescues or protects he who needs it, but unless we also add a meta-play of running X-Com, my character shouldn't need to care about funding, politics, etc. (they will inevitably have opinions about those things, in the same way they will care about the weather).
Scale of the Plot – 2-3. I'd rather feel that our actions are having an important effect, all told, on the world, rather than just being one more cog in the process. I'm fine with starting out as the latter and slowly working our way up, but I don't want to end up sitting behind a desk commanding armies.
Tone – 2. Dark is fine, but some kind of light at the end of the tunnel that I can be reasonably certain is not an oncoming train is necessary. I will take RL as my depressant pill, thanks.
Lethality/Difficulty – 2, I think. On any other game 3-4 would be my speed, but X-COM is X-COM. Just lets not go all the way to Dwarf Fortress level of insanity, please.
Permanency of Death 1 - 2. Like in X-COM, give us a chance of surviving even lethal injuries. Sitting out missions won't be fun (unless we start using alts), and I intensively dislike permanent injuries, but if you can figure out a way to make almost-death a set-back, but with a chance of recovery, that would be perfect.
Pacing – 2. Every 5 or so encounters seems to hit a sweet spot in PbP, I believe.
Update Frequency – 1. I'm good with multiple updates per day, if my turn comes around that often, and I'm good for twenty back-and-forths during non-combat. I'm seldom without internet and subscribe to all my PbPs, so I tend to answer quickly, too. I am nothing if not patient with PbPs, but I'm not going to lie to you: I enjoy it much more when things move at a quick pace.
Tolerance of Homebrew – 3. I'm fine with Surrealistik's PbP adaptation, not as much with his E10 rules (which I barely understand). I'm the ultimate fence-sitter who can see the point on both sides of the argument of 4E's "tax feats", so if you want us to give us free stuff, yay! If you think we get too many feats as it is, oh well!
Enemy Variety - 3-4. Stick with the classic and modern X-COM enemies, but feel free to introduce more varieties to account that 4E doesn't work with just one type of enemy (i.e. the way it was being done in the original PbP is perfect IMnpHO)
Why do you like X-Com? Tactical combat. Yes, base-building is fun, as is budgeting and guiding research and development (although I prefer the modern "can't sell the stuff you build" to the old "you need to turn the game into a shop simulation if you want to get ahead"), but I did not expect it in the 4E D&D adaptation. If you find a way to do it (difficult in a multiplayer), good. If you can't, I won't miss it.