It's painful to watch people defend the game still even though the man in charge of Halo admits it's bad.
This is what happens when you hire people from Infinity Ward and Treyarch and let their voices be louder than an already existing, ready to buy fan base whose majority didn't like Reach in the first place.
The mistakes that were made? Copy-paste ad nauseum from the highest selling, most played shooter on the market. Something that Halo used to be. The mistake was thinking that by mimicking those gameplay mechanics you would steal their fan base and money. Sure, people voted with their wallets in their initial interest of the game, but look at it now. More people play Medal of Honor Warfighter I bet. I heard MLG dropped Halo because of the whole Virgin Gaming deal. That's a shot in the foot that already has a hole in it.
But no, no matter what you tell people, some of them will sit there digging in their heels and deny any wrong doings. Everyone stopped playing: "That doesn't matter, as long as I can find a game I'm happy, the number of people playing doesn't matter at all". They copied Call of Duty: "No they didn't, these things aren't exclusive to CoD, they just copied everyone else (who already copy CoD)".
This is twice that Halo has failed on a main entry. Sure Frank O'Connor can promise me that Halo 5 will be better and I might even buy it to see if he is right but at this point will anyone else? Halo's population will never return to it's full strength ever again. It's done, over, the damage has been dealt. MLG picking up CoD is just further solidifying it's chokehold on the shooter world. Next generation is just going to be the same story different hardware.
343 has even bigger shoes to fill now.