To start, as someone who has been playing Halo for over 10 years, I am overall very impressed with what 343 has done.
The multiplayer mechanics (the progression system, ordinance drops, loadouts) have shaken things up enough that this may be my favorite Halo as far as the actual multiplayer gameplay goes.
That being said, there is some imbalance here. With 6+ people in your party, the only matchmaking you can do is Big Team Infinity. As fun as that is, my friends and I would really like to be able to do more gametypes without going to custom games. As fun as 5v5 CTF is, 8v8 is even better. Hopefully when new maps arrive (at the latest), this will be rectified. Ragnarok feels like the only "epic" map available for big team right now.
All-in-all, that's really my only complaint so far. Hopefully 343 will continue to show that Halo is in capable hands as the months progress.