George Takei made a comment about Shatners personality on the David Letterman show recently. He said that after all these years Shatner still pronounces his name wrong. Even though he has corrected him many, many times, Shatner just won't get it right. It's that attitude that Doohan didn't like. He was once quoted as saying "I like Captain Kirk, but I can't say that I'm very fond of Bill". Doohan would not even reprise his role as Scotty in the Futurama spoof "Where No Fan Has Gone Before". His response was "No way!". The producers believed it was because he thought he would have to work with Shatner again.
As for Doohans personality Wikipedia has this to say; "Doohan was considered by some to be one of the most giving and affable stars of the Star Trek franchise".
Actually the most well liked according to Wikipedia was Deforest Kelley who was mentioned as the only actor to have had no fueds with anyone on the show.
Unlike Mr Doohan, marrying a 17 yr he got pregnant when he was 54 (they married when she turned 18, still in middle of the pregnancy)