You post a comment or a picture and it's not consistent at all for all users. Comments will appear in the wrong order (responses before questions sometimes), or people won't see the pictures you posted until 4 to 5 hours later. With Google you just don't get that kind of things, ever.
Maybe it's only a problem for facebook due to its scale, maybe the replication rules they have set are a bit more strict and they will delay the process on purpose to avoid unnecessary writes (in case users want to delete content a few minutes later)
Then as a more psychotic/personal taste matter, I love the way MongoDB documentation is presented. Cassandra's documentation on MoinMoin looks so 2002 but that's something most people will live without any problems.
Please don't take my opinion about Cassandra seriously because I've not tried it at all. In our shop we tried Hadoop's HBase, then we tried MongoDB and since it worked for us we didn't keep trying (mostly because of what we think of Facebook as a technology company... php... their joke of an android api which made the news... our bad impression of hip hop [so long and so much resources to compile, at least when it came out], that when we saw Cassandra being used by facebook we just said no way, it's reputation was tainted by facebook in our eyes)