Fun spending other people's money. In addition to the great ideas above, here's what I would look for:
1) by far the most important...water...and a toilet (full bathroom is awesome) for the last minute pit stops before departure or upon arrival. After having a hangar for 5 years w/o direct water (it was at the end of the hangar row), I'm amazed at how much more usable it makes the hangar--we do a lot of hangar get together and parties so this is key
2) I have the manual stacker doors and they are ok. They roll fairly easily but I'd much rather have power bifold doors with a remote control left in the plane and the car(s). A friend has this and I'd definitely spring for this personally. I would make the door as large as possible
3) 50x60 or 60x50. No specific experience but I think I'd go with wider unless it drives up the cost of the door dramatically.
4) I have 4 panels that let light in...not really skylights (like MooneyFlyFast above)...ironically the leak that I do have has nothing to do with these panels
5) I'd put plenty of power (at least 1-220) around the walls and overhead lights as pernitted by code or the airport authority...I've heard of a lot of weird rules on this one
6) seal the concrete floor with whatever solution you choose as soon as the concrete is dry enough.
7) flat screen tv's for sporting events, couches, satellite TV, frigerator