Firstly, I don’t care what you think if you’ve only played non-Halo and non-FPS, and this is your first time in the FPS gaming world.
I have defeated Halo: Combat Evolved, Halo 2, Halo 3, Halo ODST, Halo Reach, Halo Anniversary, Halo 4; all on Legendary, solo, and the majority of which was done without an internet guide for legendary.
I have also defeated Call of Duty: Modern Warfare 3 solo on Veteran.
I won’t get into multiplayer, but I never claim to be the best in the world and wouldn’t even if I was because of morons.
The following are my opinions.
Halo 4 campaign: 7.9-8.3 out of 10.
Halo 4 multiplayer breakdown:
Maps: Everything feels smaller than Halo: Reach, yet not as fun as Anniversary maps for instance, with exception to Haven. I didn’t pay for map-packs, because you should be showing trophy multiplayer maps upon game-launch… While some of the maps may be aesthetically appealing, I also notice some odd features like bushes being half-pixelated until you get closer, and this happened in Spartan Ops as well, often being unable to see distant Unngoy unless you zoom in (problematic especially when attempting to use the over-nerfed, now essentially useless, rail gun.) I loved Spire and Boneyard of Halo Reach, their infinite expanse and gloriousness and allowed independence.
Ordnance: Ordnance seemed odd in that it seemed to be removed from some game types, while present in others. Ordnance seemed unaffected by wheelman sprees sadly. One problematic thing about ordnance is a lack of other things of simplicity; why is there no ammo, for per say, half, 3/5, or 75% of an ordnance circle? (Even if you could refill ordnance, someone with ordnance becomes a target of the enemy team; the enemy team could use this extra ammo, a double edged sword for the fool’s team…)
Weapons: The Covenant Carbine feels a whee bit weak to me, and I like it in campaign.
I loved the SAW and Storm Rifle.
Missed the Plasma Launcher, an excellent balanced weapon due to it’s odd way of attacking.
The Promethian weapons, save the Supressor and Sentinel, seem excellent. The Promethian grenades, “Purge” if I recall, need a bit of a buff. Currently they are only " I AM 100% oblivious to everything in the world please kill me" weapons, or semi-effective “bunker buster” or area discouragement weapon. These rarely kill nearly as much as any other grenade, and aren’t quite good enough for the area effect. (Note: I use explosives on every class, and even with this the Purge grenade radius is insufficient for killing unless enemies are oblivious.)
The DMR reticule resets way faster, I have mixed feelings about this, but it may be balanced now with the BR. Perhaps it should be removed, or perhaps it should be slightly more reticule reset delay?
I hate what was done with the rocket launcher, it no longer uses high-tech rockets to altitude adjust and fly in a straight line. Apparently they are now low tech and noob friendly… No more killing noob snipers unless you have the arc down very well… (Hate this feature about the tank also…) I liked the boost to the Covenant explosive weapons. Stickies seem to “track” in odd ways due to lag and “stuck” medals seem entirely too common in regicide.
The Sniper Rifle feels a bit too weak due to the reticule reset taking what for gamers feels like “hours.”
I LOVE THE NEW TURRETS! 
(Annoyed they seem removed from Dominion…)
Vehicles: The Mantis was nice for campaign.
However, the massive Mantis feels like an excuse to make people not feel bad about the Scorpion tank, which now feels like they Scorpions were designed by “short bus kids with the help of an A.I. and robotics factory.” The shells fall way too quickly. The tank doesn’t let you drive in one direction constantly and smoothly and quickly, while firing the turret at the same time. It feels handicapped and limited…
The banshee feels ok.
Both in Reach (recently?) and Halo 4, when you get hijacked you may never re-hijack someone and it is essentially a guaranteed death due to immobility… I miss being able to hijack 2-3 times until the victor finally drives off or kills the Spartan! IT WAS EXCITING… Also, again, hijacking a banshee or other air vehicle seems harder (recently in Reach also???) in Halo 4.
No Falcons/Hornets/Pelicans in Multiplayer? Really? Evidently guaranteeing you will never meet the awesomeness of Halo Reach?
Vehicles feel imperfect and I feel I want many back from Reech exactly save the Banshee. Hogs are hogs, we don’t expect them to be super-weapons, but we don’t prefer them to flip in combat or mere driving in a dime.
Lag: This almost seems fifty-fifty, but as for me I lag, while in Halo Reach I don’t most of the time. I don’t want to play H4 BTB because of this half the time, ending up raging at absolute “BS.” It seems to have less lag in other games like regicide. It often lags “worse” than I remember Modern Warfare 3 being, (and I have gotten more than 55 or so AC-130 killstreaks there, a fun and hard to get kill-streak.) This is the nail in the coffin of entire user-bases sometimes.
I have gone back to Reach. Sadly it isn’t letting me play a Deiant mappack map ever as of yet which I just recently bought, have not seen Timberline in forever, haven’t seen the Anniversary gametypes which used to exist, and I never recently at least seen the Anniversary mappacks in Reach multiplayer with armor abilities/grav-lifts and et cetera. Despite the problems I feel as though potentially plausible 343 is maybe even creating to make people go out and buy Halo 4, I still enjoy Reach.