I didn't like Ubuntu. Nothing real constructive or precise - just didn't like it. Restrictive, Gnome/GUI-centric. can't stand apt, symlinks everywhere and weird executable crap in /etc. Wouldn't call it lightweight, either. But it's hugely popular and, while part of that is promotion, many came, saw, liked, and stayed. So it's probably worth checking out to see if it suits.
I've been having fun with Arch lately, though I think I've had enough for now. Probably my second favorite distro at this point. Kind of hard to describe - based on Crux with traces of Slackishness and a bit of a weird Gentoo vibe, though it's a basically binary distro. Maybe it's all the colors everywhere.
Easy enough to figure out and get going with and playing with pacman is interesting - much more sensible than portage or apt or the rest though maybe not as technically good - it's practically better. Some nice (and bad) things to observe in the /etc files. I'd recommend it for anyone feeling like trying something else for awhile.