I imagine the three most desireous factors would be ease of accessibility, apparent invisibility, and coolness factor (not necessarily in that order). I can't think of anything that hasn't been mentioned - doors, hinges, tracks, etc. - other than possibly obfuscating the room in a non-traditional manner.
For instance, maybe put an inlaid entertainment console (a small one) on tracks, and inlay it into the wall. Obviously the console would need to be framed in such a fashion that it isn't evidently a door (maybe it sticks out a bit - it would all depend on how the 'door' room is organized and designed.
Design the tracks so that the console slides backwards and to one side (rotating) allowing entrance, and then can be pushed back into place (and then can obviously be opened up from the inside again) from the inside. You could then design (if you have the aptitude) a simple contact circuit for the TV's electricity for when the door is shut (or simply use a retractable cable of some kind so that you don't run into any problems). To the unknowing it looks to be 100% an integral wall complete with electricity. You might have to make the front faceplate on the bottom hinged (so it wouldn't interfere with the tracks), but you might use that as the release mechanism.
In other words, you'd pull upwards/outwards on the bottom, and it would unlatch the door, allowing it to roll backwards on a pivot (I imagine using 3 wheels, two on the ouside and one on the inside). Having it on the bottom like this is nice, as then it doesn't leave any evident signs of entry over time, like body oil. You could just attribute it to being kicked or something as well.
Alternatively, you could use a tensioned lever on the other side of the room (or in another room, even) which would deactivate the lever. It could be magnetically held in place, only requiring a fairly negligible amount of tension to unlatch the door. Then you could have the wheels on a slight incline to start off with, so it would roll backwards slightly out of the 'latched' position naturally.
I personally plan for a subterranian lair one day myself. The door may be underneath a slightly raised platform in my library. On top of the (carpeted, so you can't see a seam) platform I'll have a recliner firmly fastened to the floor - or maybe something else, we'll see. I'll use a rubber sealant all around the base of this platform so that no odors will leak into the upstairs. The trap door (which has a recliner and a reading chair on it, mind) will be released with a tensioned lever - the actual fastening mechanism being something like a car door handle in terms of tension, to hold it firmly in place. I'm not sure what I'd use for the lever - something innocuous, random, and cluttered-looking, probably, as I do the 'cluttered' thing well as a system of natural organization ;P.
The door itself would then open using hydraulic pistons, similar to what is used on the rear door of a minivan. I'd make sure the pistons and the door were in the right weight proportions that it only required approximately 20 additional pounds to shut afterwards, and a gentle tug would be enough to assist it in opening fully.
What would lie beneath is entirely in the realm of spy novels, but it's something which is reasonably achievable if I ever make enough money.
