I've recently completed the campaign on Legendary with my brother on co-operative mode and it took around 10 hours maximum. Here's my assessment of how things stand and what was so hideoulsy flawed as well as the few things I enjoyed.
First of all I felt at the beginning of the game as if I was utterly disconnected from the Master Chiefs struggle. Oh look here's where you were last time the ship still works ... even the bloody weapons still fire. Yet nahhh Cortana's not thought to activate a distress beacon? Where did a forerunner shield world come from? I assume it would have you know been obvious being all purple .... end of Halo 3 showed driftin towards a planet. But i don't recall it being anything like this. Oh look the Covenant are there as well. Time to splat some alien ****! Cortana "I thought we had a truce with the covenant" well ... HOW does she knows this!?!? She's been stuck with MC alone in a stasis pod before the war ever ended to my knowledge unless i'm missing something fairly huge. So thus how can she even know the war is over? Quoting wikipedia here on the Halo 3 entry: However, the force of Halo's blast causes the slipspace portal to collapse, resulting in only the front half of Forward Unto Dawn, carrying the Arbiter, making it back to Earth.
So yes the MC never got back to Earth. Never saw the peace treaty. Never saw the memorial service. All they'd know is the elites are on side and the rest of the covenant are rather well .... pissed off. Someone please do correct me if i'm missing some way they could know.
The plot seemed exceptionally thin on the ground to me. The captain of Infinity is all like "Herpa derpa derp why would I blow this uber dangerous enemy ship up with my space cannons man!??! What if it returned fire." well .... it seems to have no weaponry at all otherwsie it would have killed them already. So he runs off ... you scramble for another way. Then Cortana fails. That's relatively fine with me but then at the end the ship is assaulting Earth literally vaporising people. There are visibly like 6 or so UNSC ships in orbit and -none- even try to blow the ship up. Only a piddly laser to shoot a tiny piddly hole. So i must ask with humanity at risk how can you think it's good scriptwriting not to follow the logical course of action? These are the people who let the elites glass all of god damned Africa because there "might" be some flood there still. An entire continent but nah ... they're happy to let this fella with a shiny orange gun finish the Earth off instead while they watch from their capital ships bridges going "Damn man look at the colour it's so shiny." are they all tripping acid?
Then there's the Starfox / Death star torpedo run mission in your fancy fighter. Don't get me wrong this is by far my favourite mission in the entire game just for the nostalgia and adrenaline rush. But I do have to wonder to myself how exactly the MC first surprassed the shields to get inside. One can only assume the baddy is a bit of a spanner to have left them turned off prior to such a jump?
Mild campaign issues aside I noticed on the human installation at Halo installation 3 there was an outdoor area with visible space .... open to vacuum. No visible shielding and humans in helmets without mouth pieces. No vacc suits in sight. I must have been expected not to notice this error and when I later returned to find 20 grunts had wiped it all out I joked "Well maybe the humans realised all the oxygen should have been sucked from their lungs and just fell over dead". I honestly cannot comprehend this lack of common sense to map design. I know it's a game and all but it'd be nice to see some adhesion to common laws of physics and space. Especially in regards to you know ... gravity and oxygen. This said i found the early zero G combat exceptionally awesome in the first level so it's not all bad.
The ending however is the bit that had be merely staring dumbly at the screen going "Are you serious?". So here is how I see it. Darth Vader crossed over into Halo and force choked MC then force gripped him off the edge. Then Cortana suddenly became physical (somehow) and decided to become a digital spider and glue him in place. Then MC badly weakened climbs onto a bridge made out of LIGHT (how do you grab light particles no matter how dense?) and climbed up. He then grabbed a grenade. Missed. Bad darth vader wanabee casually staggers off the edge and goes splat. MC then grabs the nuclear bomb ..... hits the trigger to fire the device. Cortana havingly previously states "You won't have much time to get out" ..... we now fade to white. Next thing you know MC is in space having survived a point blank nuclear detonation to the face. But no it's FINE Cortana must have fiddled some control panel ..... teleported him right? I didn't see any terminals and i don't recall her being able to spawn wormholes from the digital world. THEN as if this wasn't implausible enough already MC is floating in space and immediately a pelican finds you! In one of the Halo books a spartan's jetpack malfunctions and he goes off into space. They state even IF they weren't in a battle it'd be near impossible to find and retrieve him in the hour of air capacity his suit containts. So again one must assume that the MC is not actually breathing as well as being immune to nuclear weaponry. It just ..... eurghhhhh. Please? Really? I'm intelligent you know. You can't fob me off with "He's a hero so he always survives." i was gunning for an actual HEROIC death here. But nahhhh cortana dies except I bet my life savings you'll bring her back next game as a physical being which is also impossible.
After this lovely little ending i'm treated to the legendary ending and shown a glimpse of John .... not Spartan 117. John. Presumably getting an armour upgrade. Now i'm all for this but really I bet that pissed off a **** tonne of hardcore halo fans out there who haven't read the books and don't know his personal story etc. That he's not -always- in his damned armour. So nice brave move there. Especially for the engineers ... the mans not showered in years by now.
I'm aware by this point i'm probably going to be described as ranting but there's a few final points before I finish up. To my knowledge it is never explained why the covenant are happy to help the enemies once again and why they're led by elites not brutes. I won't assume about the lore between Halo 3 since there's nothing concrete but surely the elites aren't going to go "WE HAVE SAVED HUMANITY!" then four years later decide to casually turn them all into promethean knights. Also where did all the covenant ships go when you grab your heavy fighter and pursue to Earth? Did they just .... go home for tea and cupcakes after blasting an entire space station to bits from the inside?
Now my non plot related issue. You have removed firefight. Made the campaign shoddy and 10 hours long and then made 50% of game content unaccessable without xbox live. Well .... why not cut the crap and make a pc game from the start so we can ALL use the internet based segment. I feel like i've bought a call of duty game for the storyline!! How often does that happen? Never. Because their storylines are **** and the games only made for the online aspect. Which this feels like you've done to Halo 4 with it's measly 10 hour campaign on legendary and poor, repetative map designs (barring a few nice touches).
OH and I almost forgot. In four years you trained replacement spartans? Oh right ..... because the grow from age like 4 to 18 in four years? Read the books before you write a damn game even the Spartan 2 program took years to make and they weren't in power armour and had barely any physiological upgrades. Oh and they almost all died. So yes it's totally feasable there are plenty of Spartans on Infinity only four years later. Not.
Frankly I found the gameplay good. I liked the new weapons. I thought the graphics were much improved and I liked the covenant retextures. I loved the promethian knights idea and thought the initial story while patchy was still very good. However by the end I felt so disengaged from the main charachter as I knew implausibly he must survive and then was proved correct via nuclear bomb and two face plantings through the windows of spacecraft (seemingly not crash proof). It just seems to slowly descend into a slog fest of orange and red corridors fighting stuff off to press buttons to get a cut scene then repeat the process in slightly different corridors. Oh I also -loved- the new guns. Especially the human LMG and the light rifle.
End rant! Thank you for listening to my views on the campaign. Troll them and i'll troll your face off.