Well RoR has got good libraries for image manipulation, OAuth (note that AppEngine
doesn't have OAuth built in), OpenID and Memcache.
Blobstore really only exists to work around the lack of file access in AppEngine.
The pricing is something of an advantage, but don't choose it on that alone. If you know Java (or Python) already then it's worth using, but it's probably not going to give you enough of an advantage to switch platform.