Sound dampening. Not as many places as it could be, sadly, but having to overhear only the conversations within 10 feet of you instead of every paper rustling in the entire 5000sqft office makes the open plan marginally less intolerable.
As davelnewton says below, if you really want to stand out, provide private offices. To everyone.
Ok, here's one more that doesn't cost as much: eliminate fluorescent lighting. If you can't replace the overheads, leave them off and give each employee their own desk lamp that they can turn on and off at will.
Must-haves: properly adjustable chairs (depth as well as height) and hydraulic desks. One size fits all desks are anything but. Your tall and short employees will have much better concentration and less RSI if even when sitting they can adjust their desk to a height appropriate for their own body. (Example: I can sit properly - without toes dangling - only at a desk 6-8" lower than the standard. The giraffe of a dude who sits next to me needs his 4" above the standard just to get his knees under it.)
