Pokemon Gen 4

Hey guys, I recently bought a cheap DS lite and the only games I have right now are Professor Layton (1) and Pokemon Pearl, and I already beat Layton. I haven't played Pokemon since Silver in middle school, but I thought I'd give the new (relatively speaking - I know I'm two years late) games a try. What I'm wondering is, what's changed besides the graphics and the Pokemon? I want to do a little bit of research on the new mechanics before jumping too far into the game. I don't care about competitive battilng or anything, I just want to know what to look out for / look up. I know Pokemon have a 5th "ability" or a passive move now, and that they can be entered in contests or something. Can I still teach two Pokemon every HM in the game and train a team of ~3 Pokemon to clear every battle in the entire game, or is it more challenging now?

I'm probably also going to buy Soul Silver when that comes out, if this is good.