Do you feel your current distro is not "so cool" like before? do you want to try something else? you have to much problems with your Distro? maybe is time to switch to another Linux Flavor. Here are my 10 cents about why you should consider to change to another distro:
1.- You Distro is very cutting edge
Distros like Fedora or OpenSuse are very quicly on they releases, sometimes you even don't finish updating and fixing your drives and another release is coming. This sometimes is frustrating, but this is good in another hand. This distros help to push new release on the enterprise side, without this distros the new improvement (like compiz in its time) will delay to much. If this is your case or need something more stable, think to try another distro.
2.- Your repository is terrible
Most of the distros has great package manager, but the fact your package manager has a great gui and are easy to use, this doesn't mean its the right to you. If you can't find your software and you need to compile everything you need or you need something else, switch to another distro.
3.- Your distro make the install process a true hell
Few months ago I found a great photo editor and I try to install it on 2 machines, one with Ubuntu and the other in Mandriva, in Ubuntu the install process was easy, but in Mandriva... it never happen. The reason was simple, the software wasn't available on Mandriva repository. Maybe what I just need was some more time to compile it or a little more search, but there're sometimes where time is a luxury and you need it ASAP.
4.- You distro use to much commercial software
Many linux purists will never use a commercial software, but let's say it, not all the good programs are open source, in fact a very small part of the software is GNU. Commercial software has some good things and other not so good, like the ability to change and customize the code. If your distro use to much commercial software think to switch to another distro
5.- Your distro don't offer support
Well, in this point let's talk about enterprise support not personal. At the enterprise level the support is critical and if your distro doesn't have or have a very poor support, don't wait any longer, change to another. Red Hat, Suse, Ubuntu... just for name some, they have a great support
6.- Your distro doesn't support the new hardware
This is the other hand of the cutting edge distros, the oldest distros. Many distros (for example debian) support to many different architectures than any other distro, and this is good and bad, bad because the new hardware is not supported. I love Debian, but when I got my new laptop I realize the wireless wasn't compatible, my graphic card... and I pay a lot money for it... sadly I have to left my distro a looking for another with the newest hardware support and then I found Fedora.
7.- Your distro is not so safe like you want
All the distros are maded in a different way, some distros focus on the development platform, other in the graphics and other on the servers. If you want your linux in your server, start looking for a safer distro. CentOS, Red Hat, Suse, Ubuntu and Trustixel are safre than linspire, because they are oriented to that.
8.- Your distro is to old
Many distros has a long way to the next release, the good is they are the most stable of all, but the other part is the software, drivers and the look is a little old. If you want the newest effects, software and the newest linux tech, change your eyes to another distro.
9.- Your distro doesn't support your new CPU
Many users use the Intel or AMD proccesors, but many other use too the PPC, Xscale, AMD 64, Intel 64, etc. not all the linux distros support different architectures. Debian is one of the fewest distros able to support almost all the architectures. If your distro don't, then switch.
10.- Your distro is boring
Belive it or not, many of the windows users switch to Linux because they stop finding what they look for... or just for fun. If you want a try another Linux flavor or discover something new... or just for fun, try, the road is open and the world is not enough.
I hope you enjoy this post, any opinions are welcome. EDIT: I don't want to start the eternal war of Vi vs emacs and this post is intented to be just a guide :D