if I've been told my history right, the reason GT blows such gigantic chunks is that Akira Toriyama didn't have anything at all to do with it. I'll retell what I've been told, but don't treat me as any sort of authority on this, I very likely could be wrong.

Toriyama was busy doing the manga (which originally was planned to end with goku dying on the planet namek). The anime caught up with the manga during dbz which explains its bad pacing and the notorious full-episode powerups -- particularly around the end of the frieza saga. Toriyama didn't think the manga would continue after that, having effectively killed off Goku and elegantly wrapping up a retelling of the super-saiyajin legend (being consumed by his own power in the end), but the fans ate it up, the studios ate it up, and he was making fat cash, so he continued with the androids/cell arc and the buu saga. But eventually even wanting money has its limits, and Toriyama decided to stop dragging out the inevitable and wrapped up DBZ. DBZ-GT was created by the studio without toriyama's artistic direction or input, and so kind of floundered, but at the same time returned to simpler roots (find some lovable characters, find the dragon balls, fight stuff). People liked GT for the return to the basics, and for kid-form goku being there (since adult goku was rather invincible, being the readily accepted strongest fighter in the universe and all). And of course the addition of yet another super saiyajin level got the fans really into it, whether or not the original creator was still writing manga for it.

That's about it. DBZ did pave a lot of roads, and hook a lot of people. You have to give it credit for that, no matter what you think of the series itself.