Well 2d opengl is somewhat of a silly term anyways since no matter what your basically always doing 3d graphics. It's just.. flat.
So instead of drawing an image you have to make 4 vertices into a square and texture it. The thing I had the hardest time with after doing java2d stuff for a long time was just adjusting to how opengl does business. i.e. you setup a state and render something, and generally trying to figure out what the hell was really going on.
Though I think without Cas's SPGL to look at/use I think I would still be doing java2d and just using openal for sound.http://chman-area.tuxfamily.org/
I also have a ton of "omfg this isnt working" newbie questions on here you might find useful is you decide to try and do this.