- What graphics libraries exist that are primarily used for 2D graphics? Answer: Slick2D, LibGDX,
- What makes each one special? Specifically, if a library provides a framework beyond OpenGL wrappers then what practical purposes does the abstraction serve?
Slick2D looks a bit like Java2D. Don't use it, as it is barely supported, and riddled with bugs, and unsupported.
LibGDX provides a whole pile of stuff, some of which you won't need, and is designed around its need to deploy easily on multiple platforms based on OpenGL and OpenGLES without code mods.
Pulpcore is gone. Don't use it.
- Assuming it took the same amount of time to learn LWJGL as it did a library built on top of it, how much programming time would it take to create the same graphics with/without the high level library?
There's almost nothing to learn in LWJGL; it's just OpenGL. It can take quite a while to make a kickass sprite engine in OpenGL. But you could probably do something adequate in a few days.
- What other costs and benefits should I keep in mind from the start?
Learning APIs always comes at a cost of not knowing exactly what's going on and then spending ages trying to find out how to do something which seems obvious, or spending ages trying to discover what you are trying to has a bug in it and it's not your fault.
- Bonus: Including Java2D, LWJGL, Slick2D, and LibGDX (and again assuming I've past the learning curve of each one), what would save the most time for creating simple graphics for (top-down and side-view) prototype games?
If you say know OpenGL then use OpenGL, code exactly what you need and no more, and be done with it.