Halo 4's numerical progression system unlocks aesthetics like emblems and armors. In order to design your Spartan, you are forced to play the game until you hit a certain rank similar to Reach. Reach offered a different scenario though, where you had multiple choices once you earned enough EXP. This type of progression was moderately flawed because there was a long stretch where everything was purchased and the player had a long ways until the next rank(which still didn't unlock anything). The basic purchase worked though. Taking a look at Halo 4's system, armors are unlocked at very distinct XP levels.
Now about player identity. Players craft what their Spartan looks like. Its intent is to have each player look unique. Players also craft their emblem. When you combine the two, in Halo 3(which had many less options) there were over a billion ways to customize your Spartan. In Reach and 4, this number skyrockets. Now how does player identity mesh into the progression system?
We can take a look at Modern Warfare 2, where players have an emblem and a player card. If you go on today, you'll see that a lot of people never changed from the starting card/emblem combo. You'll also see that players that do change have a high tendency to pick ones that are "funny" or reflect their preferred playstyle. Halo Reach locked certain armors away that were "cool" until the later tedious levels. Halo 4 does the same. Halo 4 takes this a step further though. In Reach you could craft a basic identity with your emblem. Halo 4 locks that away as well. I find it ridiculous that I had to unlock an emblem that I designed for Reach. I find it ridiculous that quite a few emblems were dropped from previous titles.
What did Halo 4 do right in these regards? It allowed the player to unlock armors based on how they play through commendation unlocks. It also failed by not giving those commendation unlocks any sort of identity.
Since progression unlock systems have to be on the money the first time, what am I proposing for future games?
-Have all emblems unlocked from the start. Players need some sense of identity when they start playing.
-Return to a modified Reach unlock system. Every armor piece is assigned a Spartan point value, except for unlocks through commendations/achievements/campaign. Have every armor shown at the start.
-Assign values based on the type of armor, not on how "cool" the dev team thinks it looks. That just results in people wearing the same shoulder pads because they were worth 250 000 credits. Helmets are the most visible, so either make them worth more or make more of them.
-Continue with the commendation unlock system. Give distinct helmet designs for each commendation. Have the Laser one be Gungnir.
-Give unlocks to campaign related stuff, similar to the Mk VI or many of Halo 3's unlocks.
-Tie some armors to legacy player similar to CQB or Military Police in Reach. But DON'T put them behind a pay wall like Halo Waypoint. (And as a bonus, you're allowing dedicated fans to craft their armor identity immediately as well as getting people to play more of the franchise).
Here's a bonus video from Halo 2 on player identity:
http://youtu.be/YGSuPZVgxLg?t=4m