Credits and Grade
Regardless of your preferred play style, you will receive rewards from investing your time and playing Halo: Reach. The concept of Grade is not linked to Trueskill and the concept of EXP from Halo 3 has been removed entirely. Ultimately, your Credits and Grade will determine which armor upgrades are available for purchase.
If you're looking for serious competition and ranked play, you should get a good look at The Arena