1. Enough computers.
2. Working areas close to the rooms, so you don't have to walk as much.
3. Any number of rooms > 15 should probably be split up into "pods."
4. The major resus rooms need to be big (7-8 meters on a side), and the minor rooms should be bigger than the ones in our ED--perhaps 4x5m.
I haven't been in working in the ER over here for very long but I do know that the number of rooms needs to match your ability to staff them. Staffing makes all the difference. I don't know if you have any control over that since I don't know the system to building an ER but I do know that staffing is the biggest issue I have seen so far. :)
Keep the public and ambulance entrances as separate as possible. Especially separate parking and driveways. We have issues with people coming to the entrance in personal vehicles and blocking ambulance spaces. If you have control over the layout outside, make sure the ground is as level as possible, and make sure there is room for ambulances to move, even when all parking spaces are full
Well, then, you'll probably need to add a few things that weren't in the original plan. :) Security areas, more generators, and the like. Take out a few things, too: fewer windows.