It's not as hard as you think. The handframe on a violin (distance from 1st to 4th finger) is almost identiacal to bass, the difference being on violin it's a perfect 4th and on bass it's a whole step. Bowing is similar. Reading music for bass takes some getting used to as the violin open strings are 5ths apart, so every other space is an open string. Bass is tuned to 4ths so the open strings alternate lines and spaces. This causes some embarrasing misses when the music goes up by a 5th and the former violinist just jumps up a string, missing the intended note by a whole step, but in my experience it's easier for violinists to switch to bass than it is for cellists. At least, my top bass players have all switched from violin.
Yeah, positions are different, but to be honest many bass players don't think about what position they're in. There's two schools of thought, with one (used in Essential Elements) treating the positions much like violin, and the other (Francois Rabbath) using the natural harmonics as guidepoints for positions, so what a violinist would think of as 3rd position, the Rabbath system would call 2nd position. I've seen one excellent scale book for bass that avoids the conflict and just calls it a "new position".