I think Halo CE had everything right, it set up the standards of what I want in a Halo game. After all the thrill of playing a new Halo wore off, I was kinda lukewarm to Halo 2 but ended up liking it increasingly more with time. It's probably my second favourite Halo game and I think it innovated the series in a right way. A lot more story was told, the split campaign, the cinematics were still well made and didn't fall into cheesy movie-type cinematics like Halo 3 (occasionally) and 4 did (in a more aggravating manner). I think my biggest complain with Halo 2's campaign is how they cut two missions out, well they just didn't make them.
Halo 3 did fairly well too. It did some nice innovation, particularly with the Scarab fights, and still had some pretty huge and long missions with the steady pacing that I liked so much from the franchise. But, some missions did suffer from a too fast pacing like Sierra 117 or Cortana for example. Some cinematics were also unnecessary and felt too movie like, taking you out of the game, particularly the last cinematic. I'm in the opinion that cinematics in video games are supposed to be quite different in nature than in movies. For one, if you don't show the main character or one of the main ones, they should be shown within seconds. You play in the shoes of that main character, they are your link into the game. If you show a cinematic without MC or the Arbiter (for Halo 2) close, you feel like watching a movie rather than being part of a video game universe. Immersion is one of the most important aspects of video games. So no fancy camera tricks either, the camera should be relatively steady up to a certain point. That's a problem I had with Halo 3's last cinematic in particular. That, and it just shows too much. I don't want everything to be shown or explained to me, it was completely unnecessary and often lets place for an underwhelming conclusion, as in Halo 3. Last big problem in Halo 3: how the hell did the Arbiter went from having him playable for half of Halo 2's campaign to merely being an occasional sidekick with barely any influence in the story? The conclusion to his story Ark was pretty underwhelming.
ODST was very welcomed and nice. As far as the missions themselves are concerned, they were not that great. They were nice but felt a bit like cheapened versions of experiences we already had. The missions were kinda short so the Halo ''formula'' was more apparent and felt a bit lacking. Nonetheless, there were some quite neat bits and the overall pace of the game was very well done. I really like the hub city, walking by night and piecing together what happened since the beginning of the day before. I was always fond of these ''you wake up after everything happened and walk around trying to figure out what happened''. And I was also always very fond of these ''occurring over a single day (or two)'' games, it feels pretty damn immersive and epic to a certain way to me. The cinematic style was also very restrained and really made you feel like in the feet of that particular ODST. The drop at the beginning was particularly nice, it didn't just last 2 seconds, you felt like you were crashing down from orbit.
Reach did a lot of things right and others wrong. For one, it fixed the problem I had with some Halo 3 cinematics. Gone were the movie-like ones, back to a more simplitic one following the protagonists. It had a somewhat Halo CE feel. Well, the game had a pretty nice Halo CE feel in a lot of aspects. But I think my biggest complain with the game and one that I feel highly disappointed towards, is the reckless pacing. Even if the cinematics feel more video game-like, I almost felt like playing an interactive movie at moments. A couple of missions had the optimal ''Halo pacing'', like Nightfall or New Alexandria for example, but a lot of them had such short segments that it was truly bothering, a lot more than these couple of fast bits in Halo 3.
For one, Winter Contingency is just so short... It's the first contact mission, we should be stressed about the situation and trying to defend ourselves, find people and organize something. The environment was a pretty cool one too. After the contact we just ride around a bit, defend a base and clear it in a matter of minutes. It should have been a bit like the Halo mission in CE. The environment was nice too and you had time to walk around and be immersed while trying to organize a resistance. Same problem with ONI Sword Base (why is the interior bit so damn short?) and Long Night of Solace? We clear the beach AND the launch base in a matter of minutes... And then the Covenant vessel segment is disappointingly short too. Truth and Reconciliation had it right. You took quite a bit of time to get underneath the ship and clear the gravity lift, only to have you wandering around the ship trying to find the captain and getting out. Part of the reason the mission was so damn fun is in its length, it feels all the more epic. In Reach, it feels like it's nothing. Heck, it's also part of why Halo CE was so good: it felt epic. And one way of achieving this, is to have truly big and long missions, to make you sweat a lot on them, intertwine it with a good dose of storytelling and make you feel like you truly are progressing. It's also why missions like The Ark and Covenant in Halo 3 are so enjoyable and rewarding.
Last thing about Reach, other than the couple of very neat and innovating bits like in Long Night of Solace or the Falcon thing in New Alexandria, Reach is basically a step backward in level design and scale. There's not a lot of big areas and if they are a step up it's not by much. It's basically the very same experience but prettier. There's not even a scarab fight like in Halo 3. Hell, level design wise, Halo 3 was more impressive! It achieved more to step up from the past game. The Sabre fight and Falcon thing are just different ways of doing things, it's not amping up the core Halo experience, which is something every new game in a franchise should do.
Now Halo 4 is a weird one. On one hand it really manages to fix Reach's problem on the scaling thing: longer missions which truly give you a sense of progression and immersion. On the other hand, it steps back in terms of cinematics (and story telling) to give a more movie-like experience. I thought the very first cinematic was abysmal. Incredibly cheesy and gave me shivers of embarrassment. Like most bits between Cortana and Chief... I wanted to close my eyes and put my hands on my ears. (there's also the Halo 4 title appearing after the first mission, like in movies... but it's a game...)
So, the missions themselves are kinda well done, go back to the pacing and Halo experience I like but they are not without their problems. First, the new enemies and weapons are incredibly boring, they are not fun to fight. They're not particularly original either, they feel like they're from the Mass Effect universe and the fights are incredibly bland. Kill the engineer-type robots, headshot the small ones and keep the big baddies for last. Really, the moment you get the recipe you'll notice that if you diverge from it the experience is more frustrating than fun. Second and quite important thing, there's no innovation at all. It feels like doing Halo CE all over again. Well the story truly is equivalent, but there's nothing Halo 4 does to build on top of the core Halo experience, nothing.