Building on what others have said, I think it's paramount in a big, flexible, collaborative space with folks running around doing all kinds of interesting things to make sure it is clear to the first-time visitor what they can do and where they can go.
In a lot of cases, that means a fair bit of very clear signage. Have fun with it--make big labels of the sort that joebenfield suggested or big arrows pointing to the service points or technical resources or designated activity spaces. Use color to draw the eye--paint is cheap, and the place has to be painted, right?
In other cases, the organization of the space itself will help. Try to think like somebody who will be walking in there for the first time. Where should they go first? Who should they talk to first? Is that obvious no matter what else is going on? Will they have to walk through the whole room to find staff/direction? Do what you can to assuage "DH lab anxiety."
Think about the service model, too--an imposing fortification of a reference/information desk probably doesn't lend itself to a nimble DH workspace, as Bethany said. But some kind of permanent desk might be helpful as a "go-to" spot. On the other hand, can it be staffed by people who are clearly willing to get up and move around to talk to those who need help or a consultation? Or is there a way for folks to signal that they need help but otherwise leave the staff to their own devices? Those are more service philosophy questions, but they have an impact on the design of the space (just make sure it's clear to the visitor how they should interact with the staff).
And related to the points about behind-the-scenes staff support, think about how the staff might grow (we can dream, right?) or change in the next several years. It's tough when a staff space is designed for the minimum number of staff and then has nowhere to grow if personnel increase. And personnel in this case isn't just permanent, full-time staff; remember that lots of grad/undergrad student workers may be used in place of FT staff, but all those bodies need a place to work when they're there at the same time.
I love the ideas being thrown out by everybody. Flexibility and comfort are key for the whole thing. And power plugs!
