Halo 3 was alright for me, but it just didn't live up to Halo 2 and it was mainly down to stupid design choices Bungie made.
A great example is getting hijacked from a Ghost, in Halo 2 you simply got ejected from the vehicle and it was fine, not unbalanced in anyway. Then along comes Halo 3 where Bungie decided to 'fix' it, so instead of simply losing the vehicle you also now lose all your shield AND get stuned so you can't move for a few seconds, which basically means you're dead. I mean it wouldn't be so bad if it was just losing your shield or just being stunned, but both? So in summary the enemy gets your ghost, while you are forced to sit there while they get a free kill
It's the same with the beatdowns where all someone had to do is shoot 5-6 AR bullets into you and melee and you then lose the remaining 40-50% of your shield and die. Again just another balancing issue that wouldn't have been such a problem for me if that melee just took the shield off first and then you had to hit/shoot them again for the kill, it would also make sense since the shield would take the majority of the force meaning it wouldn't be an instant kill. (There was also the issue of hitting someone first and then them hitting you after and you're the one who died because they had 1% more shield

but they did actually try and rebalance it so it isn't as big of an issue as it was).
The map design also wasn't up to much in comparison either, Halo 2 had classics such as Lockout, Ascension, Midship, Waterworks, Zanzibar, Ivory Tower, Burial Mounds and Headlong. Halo 3 had some decent maps but nothing in comparison to Halo 2's, hell most of Halo 3's decent maps were just remakes of Halo 1 & 2 maps
I know it is all down to peoples tastes, but the vast majority of Halo 2's maps were fantastic, I hardly ever found myself thinking "OMG not this **** again" but with Halo 3 not even half of the maps are that great and the ones that are good is because they are remakes.
And incase someone hasn't figured it out yet I am talking about the MP.