One more thought for room dimensions; for music listening I prefer higher ceilings, but for movies I find that making the room wider seems to be a big help. Bipolar or quadpolar surround speakers help to make the room seem wider, but pushing the walls a bit further apart definitely helps too.
Also, having something other than a concrete slab for your floor makes it easier for the subwoofer to shake it. As a corollary, people who enjoy peace and quiet should not be encouraged to live above or below your HT room
Finally, at some point I'm hoping to move my HT out of the loft into a purpose-built room in the (currently unfinished) basement. When I do, I will be angling the wall where the screen is located in order to get away from parallel side walls. That is definitely not essential but if you're thinking about a purpose-built room it's something to consider.
Finally, take a look at some of the sample rooms in the Rives Audio site to get an idea of what is possible - some of those rooms are spectacular and I don't think the changes really added that much to the cost. Depending on budget, you might want to consider getting Rives involved.
