I've just always viewed DT as one of the bigger (success-wise) prog bands of the '90s, and it just seems odd to me they are not on here.
As far as similar bands go, I've always lauded King's X. KX has many different side projects, including...
Platypus (Ty Tabor, guitar, of King's X, John Myung, bass of Dream Theater, Rod Morgenstein: Drums, of Dixie Dregs, Derek Sherinian: Keyboards, of Dream Theater)
Playtpus is not on AM, but I think should be, as should KX. Now, KX is not prog in the vein of Yes or KC or Rush, but it is miles and miles beyond radio (ever heard them on the radio?). Their music is very intricate at times, very emotional, with dense and difficult progressions and transitions. Their influences are Rush, the Beatles, and Zep or Black Sabbath (meaning they are one of the five best riff-writing bands ever IMO). They have no keyboards. Just great three piece hard prog rock. They have tight three part harmonies, and are all unbelievalbe musicians. They made VH1's Top 100 hard rock artists countdown at 87, higher then KC and Yes (both of which barely fit the category anyway). GREAT BAND! They've guested for Pearl Jam, Dream Theater twice, and Galactic Cowboys, among others.
Find a copy of FAITH, HOPE, LOVE or DOGMAN somewhereand let it grow on you (as most of this kind of music has to do). Songs like "We Were Born to be Loved," "Pillow," and, well shoot, all of them. They stick with you. They are my favorite band for the past two months. I'll shut up now.
http://search.yahoo.com/bin/search?p=king%27s+x