The best part is the delayed reaction and the coin-flipping process they use to determine it. It's not like you put them up and they get rejected right away. I'm helping a client try to get their PPC program under control and, as part of that, I've begun managing four figures of keywords in adCenter in a bunch of adgroups. We'll have a keyword that's been operative for a month, then I'll go make a change to the URL structure to change the tracking and three days later I'll get an email that the keyword itself has been rejected - the same keyword that was fine. Not to mention that they did this to an entire account a couple of months ago that, after a lot of stupidity on their part (I've bitched about this before profusely - admitted error claimed to be fixed turns out not to be fixed and then they decide to blame the client), has now been rebuilt under a new account using pretty much a subset of the same exact keywords and, in some cases, ads as before and pointing to the same content on the same domain. Their absurdity goes beyond keywords, they treat entire accounts this way.
They're late, inconsistent and their support is useless beyond the very basics. I hate adCenter with a passion and the only reason I didn't request the client to scrap them is that they're a good test platform for some strategies and they provide 10-20% of the traffic and conversions that AdWords does (YSM does about 60%). I'd like to hope that adCenter will get their act together, but judging by other MS efforts it could be several versions. Hell, if they could just stop trying to make their interface so fancy that it's impossible to do anything efficiently then I'd be happy.