OK just went through the game on Heroic. Gonna try Legendary this weekend. Some thoughts:
The battle rifle is uber pimp. So long as you don't use it against the Flood. I dumped half a clip into one of those things and it still wouldn't die.
The shotgun seems weaker now.
Speaking of the Flood, they're scarier looking, scarier sounding, they drive anything they get their hands on, some now have shields ( the elite version) and they now make an attempt to dodge your shotgun blasts.
Hijacking a Banshee in midflight is so cool. Suffice to say, it was not an easy thing to do.
The reason why Hunters are so much more difficult I discovered is because, their cannon fire is in more of a beam now, so you don't want to touch any part of the projectiles wake. Their shields have a farther reach. They almost immediately bring their shields up over and onto your head if you side step them and try to shoot their back. The battle rifle does not have the effect the pistol did on hunters in Halo 1. You have to work on timing a little more now.
Treat charging Brutes as you would with a charging Gold Elite with a plasma sword. If you don't have a plasma sword, you can jump over them ( you can do this with a sword elite on Halo 1 as well). Needlers work well against them.
As an elite, do you have the ability to say "wort wort wort"
Elite speak in Halo 1 is regular english sythesized backwards. "wort wort wort" is actually "go go go".