Well, there are generally 2 kinds of RPGs: linear and sandbox. Most MMOs go the linear route.
I've got my Sith Sorcerer to level 13 right now. Enjoying the storyline so far. The first Flashpoint (group instance) dark side has is doable at level 10, and it was very fun and cinematic. Not too difficult, takes 30-40 minutes, but never gets boring (unless you hate the conversations, but then why are you playing?). I've done it a few times now. Hope the other Flashpoints are as entertaining. We only had a 3 man group, so I used my NPC to tank (a full group is 4 members, no NPCs at that point), while I healed it when needed. Worked well enough. We were a little overleveled though.
Before I tried that, I checked out the PVP. The PVP warzones seem like they actually reward you too much. I did 2 games and went from level 10 to level 12. That's 2 games, no more than 15 minutes each, roughly 6000exp and 1000 credits both times. I did win both, but I hear you don't get much less for losing.