When I first started gaming (at University) we used a small study room that had a table with enough seats for six people, and one entire wall was a dry erase board. It was intimate and lacked distractions which helped keep everyone very focused on the game.
I've tried numerous different environments since then, some successful, others not. Comfortable seating, lots of table space, near-by amenities such as a bathroom and kitchen, and a lack of cluttering distractions will suffice.
My dream room is actually two adjoining rooms, both rather intimate in size. They are both rooms totally dedicated to gaming and serve no other dual purpose. In the main room there would be a large, heavy table (like a conference table) in the center of the room. This would be the main gaming table. Where the GM sat there would be a console to control lighting levels and music volume (did I mention the speakers built into the room?) as well as a small side table for private notes and such. Also, at the GMs end of the table there would be a cut out under which a computer was mounted at an angle so that the GM could look down through a Plexiglas shield on the table and clearly see the screen. This way the table surface would remain clear. Ultimately it would be awesome to have a dummy terminal similarly attached at each seat location for the sending of private messages. I would have a minifridge/microwave oven in an alcove by the door (so they wouldn't be visible from the gaming table). In the adjoining room there would be a second table with padded stools. This is the prop/miniatures table and when scenes were to be acted out with miniatures the players would gather at this table for the action. This way complex terrain could be pre-built and left until needed. There would be large cabinets in this second room which would contain terrain components, hordes of painted miniatures, gaming mats and assorted props.
I would want one full wall of the main room to be covered with a tapestry size map of the gaming world we're playing in (so the players could track their travels) and the other walls could have close-up maps of regions, cities, colorful charts with theological time lines, tidbits of historical curiosity, character portraits, etc. Space would be left for players to post their characters maps/notes and such. And a goodly sized dry erase board would be mounted in each of the two rooms. I would hang drapes in front of these boards so things could be pre-written/drawn and revealed at the appropriate time. The only other decor would be heraldic devices from the gaming world posted about.
That covers it!