Halo 4 tried to chase Call of Duty.
Problem number 1) Call of Duty is falling, people are getting tired of same repetitive gameplay elements and so that formula is no longer a winning formula. It may even be the case that an up to date Halo Combat Evolved is what people are after, but that isn't going to happen, as Anniversary didn't go that route
2) The elements that got people addicted on Call of Duty weren't good in themselves, just somehow worked well together. Killstreaks are a fundamentally flawed concept. There is no way you should be rewarded MORE for doing well. That defeats the purpose of having a competitive multiplayer game. Now, in some ways it works on Call of Duty because it is pretty much assumed that half the time the team will have a UAV up, but on its own it is a very flawed idea. What it should be like is when people are doing well, they either receive a boost that enables them to perform well, if they have the skill. For example, an ammo refill. I can't think of that much at the moment. THey should not receive superior weapons that require less skill to use, more health etc.
3) Rubbish campaign. The campaign had a very weak plot, with a boring narrative structure, overpowered characters, and unfeasible scenarios. Sure, Halo has never been big on plot, but in this game they tried to actually develop characters, and it kind of worked up to a point, but only if you ignored the god-awful writing and direction that ruined the moment.
There are many more flaws with this game specifically, but I'm bored already at this point. In short, most of the updates that they released for this game should be reversed, they need to rethink their approach to the campaign, and they need to realise that Halo 5 needs to be a decent COMPETITIVE multiplayer game. I.e. it needs to be the game that all the hardcore gamers play again, like 2 and 3.