From what i understand, itunes match is separate from icloud. It doesn't count towards your icloud storage. Neither do apps themselves. You can download or stream from itunes match.
For icloud, it's just photo roll, app data, docs, email, settings. That "app data" can be a lot though especially if you use apps to load..well...data with. Example would be a comic book app with your comics being extra data. Photo roll can be a lot as well. This is not to be confused with photo Stream which does NOT count in your icloud storage.
If you buy a 32 or 64gb phone, you'll probably tend to require more icloud if you intend to use it as backup. Too bad those owners don't automatically get more free icloud space.
There is talk that apps like angry birds can use icloud to transfer your "data" or progress from ipad to iphone apps. Start a level on one, continue it on another device. You may have to use icloud to get this.