First off, I learned guitar when I was 9 and moved over to bass at 11. I've also played trombone and tuba in the school bands/orchestra, some percussion, and lately picked up mandolin and am strengthening my keyboard technique. On guitar I'm not a shredder, I prefer the bluesy style Bard2dbone brings up: lots of bends and slides and stuff. My primary instrument is electric bass guitar.
"Play guitar on bass" means playing the exact same guitar riffs and whatnot when playing bass. (The bass line doesn't always follow the guitar; doing this just shows that the guitarist has never listened to the bass line.) Also, maintaining the "mentality" of a guitarist. By that I mean not observing the typical roles of the instruments in a given song, i.e., trying to play the solo on "Freebird" instead of laying down a groove. At its worst, having no concern for the sound of the group and stomping leads over others indiscriminantly and inappropriately for the given song.
Bassists being clueless about guitar? Sure, the major 3rd interval between the G and B strings kind of messes things up for someone who's used to having everything in 4ths. (Same thing happens when bassist/guitarist picks up violin/mandolin and has to deal with 5ths.) Also, a bassist with absolutely no guitar experience wouldn't know the chord shapes nor have developed the skill of playing them effortlessly. (OTOH, guitarists always seem to want to try full barre chords on bass which is never pleasant either.)
There's also the same problem with instrument roles. In classic rock, for example, the bass may be playing something as simple as quarter notes on an open string while the guitar is off soloing. So first the bassist has to learn the guitar solo and then has to learn any techniques associated with it. It's a fairly steep learning curve compared to the guitarist who just has to switch over to playing quarters on bass.
Alexander and s.m.80808 bring up a good point that learning multiple instruments can give you a better overall point of view on how the different parts fit together in ensemble playing. Even if you just listen to the guitar or keys or drums or vocals all the way through a tune as intently as you do the bass when trying to cop the bass line you should get a better feel for how things fit together. At a jazz clinic I heard the drummer remark how he learned phrasing from listening to Louis Armstrong play trumpet. In general, though, instruments that typically play a supporting role are less susceptible to being caught up in their own little world.
Back to expected roles, I'm sure everyone has seen ads from rock or classic rock bands looking for a bassist, but "no jazz". What do they really mean? In jazz, bass can have extended roles beyond just providing bass, especially in trios lacking polyphonic instruments (e.g., sax, bass and drums). Even in a Big Band swing ensemble, though, there are more opportunities for the bass to take an extended solo than in rock (especially classic rock). For the jazz bassist, he/she may just be trying to spice up that "root note quarters" bass line a little bit, but to the rockers the bassist is just "overplaying". OTOH, in classic rock the (lead) guitarist is expected to solo on nearly every song.
This is debatable, but I'd venture that jazz bassists have a better grasp of melody (i.e. "playing lead") than bassists from other genres. Soloing is a different frame of mind than trying to play a walking line or even a short fill. Anybody here can easily come up with a bass line to a given chord progression, but could you just as easily come up with a melody for it? It's not something most bassists spend a lot of time on, but it can open up your playing if you at least try to understand melody. (Bassists that also sing lead obviously have an advantage here.)
At a recent jam session a keyboardists told me that from his point of view he could always tell when a bassist was trying to improv a solo on guitar. He said a bassist would always seem to hesitate and overthink instead of just going for it. I think that partly has to do with guitarists simply regurgitating a string of licks they've committed to muscle memory (i.e., playing by "reflex" without having to think so much).
Just explain to him you took the chord voicings from Kenny Burrel or something. He'll lap it up 
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