fairly regularly i end up discussing with people how big of a transition qt4 is. to most of these people it seems to go against everything they've known about trolltech's management of qt which they've always known as a remarkably api stable library with extremely few defects.
it took me a while until i realized (yes, sometimes i'm pretty slow ;) that most people who use qt jumped on the bandwagon during the qt3 days, and nearly all the rest don't date back further than sometime during qt2. what isn't immediately apparent with that background experience is that qt3 wasn't a gigantic change from qt2 but more of an evolution. there were six years between the release of qt 2.0 and 4.0, and of course development of qt 2.0 started earlier than that. so qt3 when it arrived was the result of many years of having qt hammered on by production codebases. this had a non-negligable impact on its stability and maturity imho.
i personally started using qt back in the qt1 days and went through the qt1->qt2 transition. things changed radically between qt1 and qt2. if i recall correctly there were revolutions in collection classes, painting, window and dialog classes, etc... porting apps wasn't always trivial (i did few of those). so when qt4 came out with all these changes and the porting requirements and what not ... it didn't completely surprise me. it just reminded me of experiences i had back at the turn of the century.
i think we all know how qt3 ended up turning out (pretty damn terrific) and i'm quite confident that qt4 will mature similarly. in fact, i expect it to happen faster now that there are more people using it and stressing on it. what makes me excited about qt4 is that i see architectural changes that have real promise.
anyways, i thought i'd share some of my personal historical perspective for those who are newer to the land of qt and wondering why qt4 is such a radical landscape compared to qt3.
(preemptive disclaimer: these views may or may not reflect those of trolltech. if they do, it's completely accidental. i don't pass my content through anyone there nor do they pass any requirements or suggestions my way for these things so i have no way to know how "on the same page" we are here. =)