A number of my friends use their playlist column just to dump albums that they like. They have a (often) large number of playlists that each just contains one album. I guess this makes sense if you want to stay in the library mentality that iTunes is built on. Considering that Spotify now has a library of its own, buy with all the music that you have "saved" across all playlists sorted according to artist, title or album, makes this approach pretty much redundant.
Some of them, including me, try to organize playlists according to genre. So I have about 15 playlists each entitled things like Jazz, Reggae & Dub, French, 80s House, and I try to keep them somewhat homogenous, though it's pretty hard after a while and I started to doubt this approach when I almost added a new playlist called Electronic Upbeat Lounge. And you don't need to mention that Jazz should probably be at least five different lists, I know.
But the most interesting way to deal with the playlists is what you could call the "mood approach". Well, mood and/or situational, I guess. These people are making lists with songs that are homogenous, not in the way they sound, but in the mood or occasion they fit. This is not a new concept, people have been making mix tapes since the 80s, but it's quite a different thing when you can do it with this amount of music so easily available and in such a social manner. In fact, the playlists that I have enjoyed subscribing to the most are the ones that are completely mood based. It gives a lot more insight into the person who made them and allows for much more interesting music discovery.