As a former paid user of Pandora, one of the items that really had the ability of aggravating me is the "xxx thumbs radio" that happened once you thumbed up a certain amount of songs... Just because I thumbbed them up, doesn't mean that I only want those songs added as a playlist. It is redundant.
The point of having thumbs ups on the channels is to get more songs that are similar, as well as hearing those songs more often. On that note, once upon a time you were able to shelf a song for some amount of time. It seems that option has been removed...
There should be a rating system that goes further than thumb up or thumb down. One that says "this song isn't right for this channel". For the moments when a song that has no business being on a channel (ie Johnny Cash plays on a death metal channel). As of right now, you have to thumb down that song, but that only tells Pandora that you do not like that song, not that the song doesn't belong mixed in with the rest.
A other idea along the same lines, the ability to fine tune your channels. If I type in 1980s, I would expect only 1980s songs to play. That is not the case. I would get tracks from all over the decades, strewn about in no specific order. Maybe an advanced search bar may be the solution. Not just searching for genre, song, artist, but having advanced options that you can choose for instance (band) (date range) (genre). Tears For Fears, 1980-1989, post punk. Only songs that meet the criteria chosen will play. Those advanced search options would cut down on songs that don't belong, say Eifel 65's Blue right now may have some similarities to Tears For Fears with the tags system that is currently part of Pandora, but has no place being played in a channel that someone created based off of the new wave style of 1980s Tears For Fears.