What I actually find lacking the most in Reach's firefight is the ability to enable skulls by set. It's such a simple thing, but it's what gave ODST's firefight that truly progressive feeling, where the difficulty increases until you're really struggling to kill even the grunts. In Reach, as far as I can tell, every set is exactly the same. Skulls get enabled by round, but all reset at the beginning of the next set. It gives Reach's firefight this endless, we could be here forever it's so easy feeling.
Please excuse me if there actually is a way to do this and I just haven't figured it out. I haven't played much since Reach first came out, and I just stumbled across this thread on accident. I've messed with the options quite a bit but still haven't figured out how to make a game that increases in difficulty after the first set. Playing classic mode (I think it's classic) with a limited life pool and default options does seem to get slightly more difficult over time, but it isn't because of skulls as far as I can tell. Do the enemies slowly get stronger? It's hard to tell sometimes.
A couple other small things...I kinda liked the grunts better when I could understand what they were saying. Yeah I know the lore and why Reach had to be this way, but it takes a bit of the charm out of it for me.
Also, I really, REALLY liked the voices better in ODST. There was something just really frakkin awesome about fighting for your life against hordes of covenant as a team of Firefly voice actors! I know Nathan Fillion (voice of Buck) is back in Reach, but his lines are nowhere near as good as they were in ODST. Plus I miss Alan Tudyk and Adam Baldwin (Wash and Jayne from Firefly). That game had some really awesome and crazy lines.
"Bip Bam in the brainpan" (always sounded like breadpan to me)
"Here's Vera" (firefly reference)
and one of my personal favorites from Buck, when pulling out a rocket launcher..."Heat in the pipe!!...that's what she said."
Good memories :)