Music theory is just 1 side of it. The different role in the band means you need to approach the instrument in a different way than with guitar. The last thing you want is a bassist that plays his bass like it's a guitar. There is a lot you need to learn as a bassist that is different than what this site has at the moment.

Technique is obviously completely different.

Check these places out:

studybass.com (lots of good lessons to get you started)
talkbass.com (kind of like the forum community here, but geared towards bass)