G4 has posted an article detailing the 'Player Investment' system. As the name implies, it's designed to keep you -- the player -- invested in the character you've created. The character you create in Halo: Reach is persistent across both the single-player campaign and multiplayer -- even appearing in the cutscenes -- and both modes will allow players to unlock new customization options. Players earn credits (abbreviated cR) simply by playing the game and then these credits can be used to purchase different custom pieces of armor. Many different tasks will yield credits, though G4 notes that Bungie is making sure to balance task so they don't get in the way of normal gameplay. Each piece of armor, incidentally, is solely for visual customization -- they won't provide any gameplay benefits a la Modern Warfare.
One of the ways players will earn cR is with the new challenge system. Bungie will create daily and weekly challenges, small goals designed to keep players interested over the long term. The challenges can be anything, with Bungie's Luke Smith noting "kill X dudes" as an example. Whether said dudes be in multiplayer or single-player, that's up to Bungie.
The article also touches on the new ranking system. In addition to ranking players based on skill, the game will also track how they play. Players can earn "Commendations" which reflect their playing style. Make kills with a specific weapon and you earn Commendations for that weapon, which other players can see. Commendations can be earned for other actions too, like driving vehicles.
Finally, Microsoft has announced that new Halo: Reach multiplayer info will be dropping every Friday between now and the launch of the multiplayer beta in May. The updates will appear on Bungie.net starting this Friday, so be sure to keep your eyes peeled (for jet packs).