First, realize that you can't create an environment that will please everyone. Then, consider these criteria, which have slowly risen to the top of my list.
- Quiet. An effective work environment needs to be quiet. To the point where you can concentrate, but not to the level where you feel like you're isolated. It's hard to strike that balance, but this is the single most important criterion, imho.
- A "water cooler". I put that in quotes, because you don't need a physical water cooler, but a place where people can nosh and talk, separately from the quiet place above. It serves as an informal discussion area. Almost as good as the monastery. ;-)
- Aesthetic "non-computer" art. Really. It makes a difference when you can look at a nice painting or sculpture that has nothing to do with computers to remind you that the bug you've been working on is not the be-all and end-all of your life.