I've heard the majority of his stuff. His solos are extremely weak in most songs, competent in some though. He's known for brutal riffs and that's pretty much it. I never heard anyone prop him up so high until he was killed. He was considered fairly mediocre before that.
Eh, I think its an exaggeration to say that most of his solos were 'extremely weak'. Very few guitarists have awesome solos every song.
Not to mention that there's more to playing guitar than solos. The meat of a metal/rock song is usually the rhythm guitar. You can be a great metal guitarist without being a technical god.
Either way, Dimebag was extremely proficient technically, and he was most definitely NOT considered fairly mediocre before his death. Thats ridiculous. He was a metal guitar god. Its the entire reason his death was such a big deal.