It has a simple unintrusive ui which is nice.
Netflix/Hulu - no user myself and I'm not sure if those services are available outside of the US/available in DE. Tab casting, as far as I understand that, is a Chrome feature. I'm a Firefox guy. Would I be able to do something useful with that device?
Right now it seems like it would be equivalent to the Pi w/ xbmc, in an arguably smaller/nicer package. I'm hoping for a killer feature that I haven't considered/noticed :)
I'm basically in the same situation as you. I already have Raspbmc set up on my Raspberry Pi, and see little reason to purchase a Chromecast. In my understanding, the "killer feature" is that it's supported by an increasingly large number of Android apps, which don't have support for casting to arbitrary UPnP targets as far as I'm aware.
Currently my main applications used by my chromecast are watchever (a service like netflix), twitch (using a third party app shame on me ;)) and youtube while I use the Pi mainly for "offline" media.
The most appealing feature of the chromecast is that it "just works" and I never had major problem with it (super easy setup, great integration into existing apps, turns on the tv once it is activated on my phone and so on). For me it delivers on the promises, which manufacturers of "smart TVs" made but failed to achieve in their products.