About to buy an ATV as well. I usually never ever buy stuff when I know it can be refreshed at any minute. For all we know, since the ATV isn't that big of a deal compared to the iPhone/iPad/Mac, Apple can update it quietly next week. Or tomorrow. Eek!
It's very possible that they simply didn't have much to add for developers, and there's no point talking about it at WWDC. Or, if they do have all new and great stuff, an App Store, and other things developers might be interested in, they could talk about it at WWDC and then don't ship it until the fall. Who knows.
In any case, like others said, it depends on your use. My main use for the ATV will be AirPlay, so everything else really doesn't matter to me that much. Maybe they'll add games. Siri. A revamped UI (probably runs on the current ones too anyway). Some sort of US-only (not me) stuff regarding cable networks. Better. Faster. Stronger. Etc.
All I know is that I currently have a new TV and no AirPlay. Despite it being very counter-intuivitve, I figure that every day I don't buy an ATV I simply can't stream music and videos to it, and that sucks. Maybe they'll update the ATV next week, but then I'll still have AirPlay on the current device, and correct me if I'm wrong, but I don't think a next version will change anything about the whole AirPlay thing.
Bottom line, at this point I just don't want to wait what could well be 6 months for an update that possibly adds absolutely nothing I would even use or really care about.