Mark Nottingham has a good post running through a few topics on which people get needlessly caught when designing RESTful applications. If you’re new to working on RESTful application design (as many rails developers are) it’s worth checking out to save yourself needless anguish.
Thankfully for Rails developers at least some of the issues he identifies will be a little simpler than they might be for people designing systems from scratch. In particular, while there are a few URL design choices (numeric IDs, other parameters, or a hybrid? nested vs. flat?) the conventions are good and changing isn’t all that hard.
And while some sort of schema definition is important to ensure that server and client can be sure they’re speaking the same language, for those using tools like ActiveResource which take full advantage of dynamic languages, there’s a little more flexibility than there may have been before.