Dane Jasper, the CEO of American internet service provider Sonic.net, made a recent blog posting outlining five evil deeds that ISPs do. Some of these deeds (or misdeeds) hurt you indirectly, like giving your information to advertisers, while others hurt you in a very tangible way, like giving your information to whoever asks.
Image by Wikipedia
The five, in order of least evil to most evil, are improper NXDOMAIN handling, clickstream tracking, ad swapping, affiliate program pumping and rolling over (your information to governments or content creators). Some of these, like clickstream tracking, ad swapping and affiliate program pumping, take your traffic and either swap out ads or direct your browser to sites and affiliate programs in which your ISP gets a cut of the proceeds. These are pretty bad in a privacy/ethics sense, but in a practical sense, it doesn't hurt you that much. The top image is an example of how one of these systems works.
What's really bad is when ISPs give customer information to law firms or content providers (think RIAA and MPAA) that improperly serve subpoenas, so that lawyers can contact you and demand money for whatever copyright violation you may or may not have done. He's not saying that complying with the authorities when it's legally mandated and actually legit is a bad thing, he's talking about all the snooping and not-quite-legal stuff that ISPs engage in because it's easier for them to just give up your data than spend the time and money to fight it.
How do you know if your current ISP engages in these practices? It's relatively easy to check the affiliate/ad stuff by mis-typing some searches and seeing your ISP does in response, but there's no easy way to tell how your ISP will react to subpoenas or requests by the entertainment industry until they do it. One thing you can do is Google phrases like "[ISP name here]gave my information" and see what turns up.
The Five Levels of ISP Evil [Sonic.net]