I will look over the 4e books, before deciding to playtest it or not. I like my current campaign and do not intend to abandon it, so I shall continue to run it with 3.5e.
4e needs bards and druids, at a bare minimum, to be acceptable. It needs Savage Species, Stormwrack, and the Greyhawk Gazetteer, to be useful. It needs a DDI with full Mac compatibility, to be enticing.