So I have done quite a bit with Java2D at this point, and started wondering: what's next? LWJGL and JOGL look like too much to just jump right in right now, so is there an in-between step I can take? If so, are there any good tutorials out there for it?

Thanks,
-Nathan

Slick will give you a lot better performance than java2d, but it pretty much hides lwjgl, so you won't learn very much about it. If you don't really care if you learn anything, definitely go with slick. It's a great library!

If you want to learn a lot, dive straight into OpenGL. If you want to be productive, pick a library that does the heavy lifting for you.

Personally, I'd recommend doing some 2D stuff in OpenGL

Good choice.

Good OpenGL tutorials: http://nehe.gamedev.net/

hmm, the Nehe tutorials? I really wouldn't recommend you use those these days, they're old and teach outdated OpenGL. If you are going to invest time in learning OpenGL you really should go straight to learning modern OpenGL. If you learn old opengl (fixed function pipeline) then its quiet a challenge later adapting to the mindset of new OpenGL (programmable pipeline) so best to just start with it.

I would recommend you use this tutorial/book 'Learning Modern 3D Graphics Programming' there are LWJGL ports of the examples in there here to assist you with your learning.