In the early-2000s like many other people I was attracted to the Crystal[1] theme/icon pack, but it did not take long for it to start looking tacky to me. I feel like KDE never really abandoned this kind of look-and-feel until Plasma was released, which is very clean and pleasant. It has discarded cartoon-ish icons and has sharper lines and colours which are nicer to look at. I am a big fan!

[1] https://en.wikipedia.org/wiki/Everaldo_Coelho#Crystal