How about 'Reliability'. Their API is constantly changing in what seems like random ways, and after updates there are always a couple weeks where everything is incredibly flaky.
Thanks for posting this. I'm having all sorts of problems right now (today) with code that worked yesterday suddenly not. And nobody answers their bug posts.
Agree. I'm doing a lot of Facebook app development lately and the API is really a mess at the moment. The documentation is terrible (and they shut down their only hope: the wiki), the API is confusing and inconsistently implemented, and they have a lot of downtime due to various bugs.
It has intermittent latency too. The new "like" button doesn't work right on FireFox either.
Off-topic: Are you the guy who made the IAMA on Reddit about your successful website? I was just wondering how you're doing and if you're still working on startups? :)