Possibility that it will get slower with use/time.
Possibility you will have intermittent long-lasting beachballs, e.g. when waking from sleep.
There are some bricked SSDs out there, usually still allow reads but no writes.
Quite a few RMAs on them, probably a higher percentage of sales than that of standard HDs. This is still somewhat of a nascent, early-adopter technology: you can get a paper cut when on the bleeding edge.
Some are not nice to your battery.
Rapid obsolescence.
If you are getting Apple's SSD, you get better reliability and consistency, but slower speed than many third-party SSDs. I think in the 2011 MBP, for example, Apple's SSD offering does not even have a SATA III interface, even though the bus in those machines support that. And even compared to other SATA II drives, Apple's offerings are slower.
For a sample of the kinds of nightmares that can occur, see these
user reviews and
this article, which describes a data-loss bug in the Intel X25-M drives.