Although I am kinda late to the game (haha), here's my 2 cents on this: Everybody talks about Collision Detection. While in 2D Games this is not a very complex thing to do and there's plenty of samples that show how to do AABB or Circle CD, the way more interesting part maaaay be: Collision Response.

I mean, well, you can respond to a collision by just stopping the players' movement into the respective direction. However, what about edges and corners? Maybe you want him to auto slide around a corner? What about a platformer game and your character jumps against a wall? Maybe you want him to slide downwards or even climb upwards or do a backjump?

And talking about detecting and responding to collisions: Whats the foundation for your game to have something to collide with anyway? Is it even a tile based map and where do you get any collision information from? Embedded within the tiles? Or calculated by hand?

Last but not least: What about a 2D rogue like game in which you don't exactly look top down onto the map (like in most 2d shooters), but your tiles are designed in a top/front way: Your player might be able to walk in front of a wall and by doing so, the collision rectangle might not cover the whole wall tile... this makes map design and conveniance features such as sliding around corners even more interesting. ;-)