A constant source of criticism for Halo 3: ODST has been the lack of matchmaking for the popular Firefight mode. You can't jump into a random Firefight game; matches must be coordinated with friends. Since it's a cooperative-dependent mode, most people would want to partner up with friends anyway, but the lack of a randomized option was clearly a sore spot that's haunted the game.
As it turns out, Bungie wanted matchmaking in Firefight, too. Bungie recently revealed why matchmaking didn't make it into Halo 3: ODST's Firefight in a post-release interview with G4.
"When we first started toying around with what became Firefight, we all knew we wanted to have Matchmaking for it," said Halo 3: ODST design lead Lars Bakken. "We also knew that would mean some pretty heavy investment on the networking side, which meant more people, and unfortunately it was quickly shot down. The Halo 3 engine doesn't have support for Matchmaking in our synchronous networking model (which is what powers Campaign and Firefight), so we couldn't do it within the scope of this project. It still bums me out, but there are always things you can't do, and you just have to move on."
Halo: Reach does not use the engine that powered both Halo 3 and Halo 3: ODST. It's built from the ground up. It would make sense that Bungie would make note of this flaw in the Halo 3 engine for their next project, but the company is remaining secretive about Halo: Reach. Bungie wouldn't confirm Firefight would appear in Halo: Reach, but their cryptic response makes it seem pretty likely, too.
"Well, it was pretty well received, which we're happy about," said Bakken. "Will Firefight return in a future Bungie Halo game? Only time will tell."