You should check out the other Final Fantasy games. This is a good quote:
Is Final Fantasy XIII the only Final Fantasy title you've played?
Final Fantasy XIV is being re-done to seem more like a fan-service title bringing back everything that was in previous titles in the series. Final Fantasy XIII (and XIV 1.0) seemed like very mis-matched odd sheep in the bunch.
If I had to compare Final Fantasy XIV to another Final Fantasy, it would be more in line of XI and XII in world/gameplay design, IV/V/VI in story/atmosphere, and III/V in the Job Design....
If you want to cut to the chase go play FF X and XII, avoid X-2 like the plague. They have a similar style of open world type but clearly can't match the MMORPG aspect because they are not. Really XII is the one I would play if you only had time for one game but X is where they broke completely away from open world game play and added a type of rail system. XIII is almost literally, if not completely, a rail game.
Just know that FFXIV is open ended and sandbox type. You can do what you want, when you want, with little penalty, other than not having an item or access to things. That you already know about from your previous MMO experience. GL, I can't wait for the PS3 release and I was really excited to play it on the PC while I could.