I agree that bass does require a different mind-set to guitar...however, (imho) there is a lot in common with a rhythm guitarist.
Tone wise, I reckon there is a lot to be gained from developing a solid finger style, plectrums just have a different attack and give a different sound/feel to fingerstyle. Slapping n popping is another topic altogether! All three techniques have a place in your arsenal as a bassist.
Feelwise, note duration (whole, half, quarter, 8th, 16th) was a topic I had to re-visit (and constantly go back for refresher lessons) and remind myself that my job was to "make the drummer sound good" or as I simply say to myself, lock in with the kick. Simple 12 bar blues lines are great example of simple & effective bass groove(s). The more I lock with the drums, the more I feel I'm doing the job of a bassist...
Here's a structured resource with LOADS of material : http://www.studybass.com/
- I also follow a few guys on youtube, "Talking Bass" (lots of bass line tutorials) & "Scott's Bass Lessons".