Earlier quoted context omitted.
Slackware and Arch Linux are great options if you're interested in exploring something a little more advanced. You'll have to put more work into setting up your system, particularly at first, but you will learn a lot more about how Linux works. They kind of throw you in the deep end, but they both have plenty of good documentation. On the other hand, if you're looking for something not too far from Ubuntu, Debian or…
Well, I started with slackware (well, actually SuSE when it was still a rebranded slackware, around linux 1.0.8ish), and I don't really want to dive into deep ends for my work machines anymore, I just want them to work and be moderately up to date. Ubuntu looked so promising at 7.04...
The machines are also fairly ageing, and don't support the composited desktop. I did test Ubuntu 11.10 beta on one, but it was so slow (with what I assume was Unity 2D) that it would be unusable for day-to-day work. All we use at work are Chrome, Thunderbird, OpenOffice and Audacity - why would we need Unity? Why can't we turn it off?
For me, Ubuntu definitely peaked at 10.04 and I'm going to be looking elsewhere in the near future.