For me, the biggest considerations are adequate room for your subject, and proper lighting. The former varies according to what you build - one fellow I know builds magnificent 1/1250 models out of a large utility closet. I build large ship models (the biggest so far is 8 feet long), so my shop is big (a move out to the country after many years of city living lead to finally being able to build my dream shop)
http://wmunderway.8m.com/workshop/workshop.html
As these days I mostly scratchbuild, one thing I find very useful are multi drawer cabinets I made to hold things like styrene strip and brass shapes - these are under my main bench where I can get to them readily.
By the way, the photos were taken as the shop was being completed. I can assure you that it has never been that neat since LOL
Kurt