Funny thing is, those are all questions that every game developer struggles with. :lol:
I'm the same way... I enjoy games, but I enjoy making them even more. I'm not a big fan of C or C++, but Java has been easy to pick up on.
For 3D models and animation, there's a few code snippets available in these forums, but keep in mind that LWJGL is meant to be an OpenGL wrapper, not a full-on game-building solution. I've seen LWJGL used for much more than games (including some great artwork recently posted about on this forum, and I've even considered using it myself to build some financial applications with 3D charts). If you're looking for a prebuilt game engine, you can try something like Xith3D where they already have the model loaders and things written for you.
For the unit lengths, that ENTIRELY depends on what it is you want to do. I personally stay clear of anything like that, as my games tend to be more cartoonish than realistic.
For building 3D models, once again, it depends on the tools you have available. I personally like using Blender3D (it's free) along with PoseRay for post-build tweaking. Blender3D also has some great tutorials online.
Fast-smooth games: Like in all things, you need to know how the underlying system works. Java is certainly more forgiving with it's garbage collecting, but if you write it efficiently in the first place, your users will notice a difference. I'd recommend getting a good Java book (I personally like "Teach Yourself Java in 21 days").
Are games a waste of time? It sounds like you're like I was at the beginning. I'm very conscientious of my time and was raised in a somewhat "controlled" environment when it came to computer games. However, some of my best memories deal with playing a good computer game down in my basement when I was a teenager (anyone remember the incredible immersiveness of Ultima 7?). I've found that I like to make games that teach people something. My most recent project was actually a children's book about a character who runs into puzzles that need to be solved. The reader is then given the puzzle in the form of a computer game that can be solved from clues hidden in the book's text. I think this blend is particularly appealing to parents who are trying to get their kids to read something.
Find your niche, something you love to do, and stick with it. It sounds like you're very much like the rest of us, and have a keen desire to want to create and build. I say, follow your dream!