Zlib is better for developers than the dual (L)GPL / commercial license that SDL uses.

Documentation comes in multiple types. First, there are tutorials, next, the API documentation is actually the code itself.

Unfortunately there are no tutorials yet for the latest development branch but only for the latest release, however, most of the tutorials are still valid and the best thing in the SFML code is that it contains huge amounts of comments that almost make any kind of documentation unnecessary.

By the way, I still praise SDL as it is a proven toolset, but anyway, I would strongly suggest you to check out SFML as I ended up with much less burden about different things since I use it.

SFML does not really have plugins as it has a far smaller user base yet compared to SDL, but it is kind of an end-to-end solution for most of the tasks needed. It even contains some things like sprite based graphics that most probably I’ll never use, but it allows me not to link that part of the library with my application if I don’t need it.

SDL has prooven great extensibility (lots of plugins) and great documentation.