A bit late to answer, but the others don't mention an interesting and feature of Pandora's curation method.
Pandora became famous when they built a model of the "music genome". They ran a massive project with music experts trained to assign every song an incredible range of 450 categories and metrics. Each song was evaluated and assigned categories and metrics like "genre", "use of [brass] horns", "BPM" (beats per minute), "lyrics/instrumental [no lyrics]", "language", etc. Artists and albums are probably evaluated/scored just by averaging all the songs associated with them.
When you select different songs as seeds, the channel is then built to achieve a reasonable similarity to those sound attributes. If you add a song, it might narrow down the range of songs matching the criteria, OR it might expand the range by increasing the variability of the seeds. For example, if you chose a marching song by John Phillip Sousa, an R&B song by Beyonce, and a metal song by Metallica, your channel would probably a wide range of music. It might even seem random. On the other hand, if you choose for your seeds three rhumba songs with the same instruments, BPM, language, and artists all born in Cuba in the 1960s and had even collaborated with each other, you would probably a get more narrow range of music.
Pandora does not explain this algorithm, but I think for all of its value, they kind of ruin it once they allow the thumbs up/down function to influence the list.
If you use a thumbs down button, the song won't be played. That is typically clearly useful. Songs like it might also not get played, but I have not noticed this to be an effect.
If you use a thumbs up button, it seems that song is given a boost to its probability of being selected. This effect gets noticeable over time. You eventually might feel like such songs get overplayed, particularly if you crave hearing new music or music you have not heard in a long time. However, I have noticed a problem of another sort. Once you thumb up a song, you are liking it FOR THAT CHANNEL, even if you might not really want to hear the song again on that channel. This can degrade the targeted selectivity of a channel, particularly if you are not paying attention and forget why you created the channel in the first place.
Here is an example. I am listening to a channel I created. It is basically a rock channel for medium-to-hard rock songs from old Guess Who to more relatively recent artists like Soraia. Anything with some pep, though preferably not too much noise. Well, somehow a 1950s/1960s simple rock-and-roll pops up, let's say a Dion song like The Wanderer. Now, I used to dance to that song at Lindy Hop/Swing dances in the early 2000s. I like the memories. So, I select thumbs up. Then, the station starts playing more songs like that. Another classic "rock-and-roll" song like Unchained Melody might come up. I like this song in the right mood, so I might absent-mindedly hit thumbs up. Well not only is that older style and not edgy, it's slow. So, songs like this now alter the feel of the station.
If you have that problem, you COULD edit your list of thumbed-up songs to fix the station (if that feature still exists), but after being a long-time listener, going through that list can be time-consuming. So, using the thumbs-up feature can lead to hard-to-fix problems, while the seeds are generally more intentional and easier to change.