The game definitely has a lot of hours in it. Most of the people I know have played 2 or 3 runthroughs just to experience the different possibilities. I think it'd be pretty hard to unlock every specialization and outcome in one playthrough, because a lot of them are polar opposite.
The Nexus community which produced mods for Oblivion, Morrowind, and Fallout 3 has also spawned a Dragon Age Nexus. I can't recommend enough going there and checking out some of the great mods.
You'll probably want to start by getting:
which is a tool for managing mods to replace the shoddy DAUpdater which comes by default. From there it all comes down to personal preference. There's a few mods I think are essential but not everyone will feel the same way.
PS. Sorry if I got that whole code box thing wrong, wasn't sure if I was supposed to make it a non-clickable link in addition to containing it, so opted for convenience.