Over the years, ok not so many I have only been using Linux for about nine years now, I have managed to try the big hitters in the Linux world, the Slackwares, Red Hats, and Debians, and many variations and this is what I have found to be true. Red Hat and Novell Distributions seem to be geared towards Enterprise systems with a lot of offerings for businesses, Debian distros are more geared for end-users, and Slackware seems to be for the tweakers.
Ok so what do I like and why. Source-based distributions (have to mention OpenSDE here) are fun to play with but hard for someone like who is based almost entirely in a GUI environment thanks to my Windows training. Debian I love as a server distro and haven't found a need to use anything else. Opensuse I love on my desktop. If you run Gnome I find you hard to use, sorry remember Windows Guy, and I tend to swing towards distros that use KDE because of this.
What I look for in a distro is simple. KDE is a must and now that 4 is stable I like it over 3. Wireless support because I use it on my laptop. A strong repository, I don't know about you but with me I have tried some of these obscure distros with weird package management, that they hope will be the next RPM, DEB, or TGZ (not sure if this is what Slackware is still using, could Google it I guess) but doesn't have support for your favorite tools, or applications, and you can't find the app that you need at that moment. Standards, ok this goes to the repository bit too, how many differant versions of anything do we need. Granted most of the distros nowadays are based in Debian or RPM (note: I didnt say Red Hat because Novell fits this category too) and so the standard for me tends to be one of these two package management system and Common Toolset.
If I were to build my own distro. So this is the part where I get creative and you can all yell at me later because I am being unrealistic. <edited due to unreasonable thinking> This section will be filled later.
In conclusion, still much to learn. However, my favourites are Opensuse and Debian, and if someone wants to chime in and give me a few suggestions I don't mind, especially source based I wouldn't mind trying that again.