For me, StumbleUpon is a great place to make connections, see great pictures, and find articles or sites that I would otherwise not be exposed to. The whole idea of SU is to share great finds with your friends and for great content to get exposure.
This concept has evolved, to say the least. Now you may be exposed to sites that are not relevant to your preferences just because it has been thumbed up many times by your friends, and their friends, and so on. These sites have been “pushed” through the system, for the sake of traffic/views. Recently SU has been trying to combat this situation by adding the share feature, and even offering advertisement etc. In addition, they are banning “circles” of mutuals that seem to be sharing, thumbing, and reviewing the same sites.
If you happen to be in one of these “circles,” you run the risk of getting “ghost” banned or completely banned.
Ghost Banned on StumbleUpon: You can thumb up, thumb down, discover… pretty much anything a regular user can do. The only thing is, your efforts don’t count. You can tell if you’re “ghost banned” by discovering a page, opening up a different browser, and visiting the review page of the site you just discovered. If it says “Discovered by someone” and not you, you’re a ghost. No word yet on how to reverse this.
Here are some things to avoid:

It seems that Digg is being a little hard on the Beaver. The 



