Advice for new jailbreakers:
1) The jailbreak is fine. You don't happen to own the one iOS device that it doesn't work on, assuming that the device wasn't messed up to begin with.
2) The jailbreak is reversible. If you can't handle the responsibility of having root access to your device, you can go back to your restricted access by simply restoring in iTunes.
3) Sometimes restoring in iTunes gives you an error. If so, you're doing something wrong. Make sure your TinyUmbrella settings aren't messing you up, and try a different USB port. If that fails, use a different computer. It will work eventually.
3) Don't install every theme and tweak that you can get your hands on. Some mods can conflict with others and cause problems. If you go crazy (like my kids always do) you will end up with a slow phone that only lasts an hour without a charge. Be responsible, since there's no team vetting Cydia submissions. Most packages are ok, but there are some that appear to have been coded by monkeys.
4) If you are on a newly released jailbreak on a new iOS version, a lot of Cydia apps are not going to work correctly. Just wait a couple of weeks, and most packages worth a darn will get updated. If you are on a device that has only recently been able to be jailbroken, expect issues for the first few weeks, too.
5) If you post on the forums asking for help because something isn't working, you're going to get the same answer as everyone else. We like to help, that's why we're here, but your phone isn't unique. Restart your borked jailbroken device in safe mode by restarting it while holding the volume button up and start trying to untangle the mess you've gotten yourself by uninstalling packages in Cydia.
6) If you start manually modifying system files or deleting them for some reason (some people do this!) and your device starts getting hot or you run out of RAM or the battery starts to suck, all bets are off. Just start over and keep your nose out of those folders from now on.
7) For the most reliable jailbroken device, start from scratch. Begin by restoring your phone in iTunes to the firmware that you want to use. Set it up as a new phone. Do not simply update your phone to the newest firmware and restore your backup data. Ignore your backup. If you have data that you absolutely cannot back up some other way or live without, you probably shouldn't be jailbreaking in the first place. A completely clean slate is by far the best foundation to start with.