In a broader sense, DT's music really opened me up to all kinds of progressive stuff, and it was a major shift for me, coming from a thrash metal background. I got my hands on a used copy of IAW, and was floored.
As for the purely technical stuff, as a bass player, I'll have to agree with Scard. The three finger technique was a big thing. One thing I found really useful on Myung's instructional video, was his exercises for developing right hand stamina and consistency using different combinations. I still use those, as simple as they are.
I also think that learning DT songs, especially instrumental sections, has taught me to think about music without always breaking it down into measures. It sounds ironic, since so much of it seems to be very theoretical (as opposed to "simpler" music). But some of those long sections were so confusing when I looked at the actual transcriptions, that I felt like I was better off learning it by ear. So I would start to see it more as a melodic line or phrase, which is really useful sometimes.