I was lucky enough to win a 360 + Reach when it came out, and had owned a PS3 since Christmas 2007. Gotta say, that game blew me away. Not sure what Halo 3/2/CE were like but goddamn, to me, Reach was untouchable. Epic story IMO, graphics seemed mind-blowing (but that might be some rose-tinted glasses), and the online was great, especially coming from PSN. After finishing the SP and a few hours of MP, I was like "insta-GOTY".
Now coming to Halo 4, I wasn't really hyped, but I bought it because it was ~$60 at launch (that's pretty cheap in Aus) so I was like what the hell, I loved Reach, so this should be somewhat good even if not by Bungie... Oh boy, was I wrong. This was the first game I played as Masterchief, but I obviously understood his significance, and the fact that he was basically lost at the end of Halo 3. So here I was thinking there was going to be some epic reveals, characters gasping, anime cut to multiple people all bewildered by Goku's MCs return. But no everyone was just like "what up dude." I dunno came across weird. Also, graphically, I thought it was really lackluster. I should preface that by saying between Reach and 4, I built a hefty gaming PC, so maybe that contributed to my reaction, but I definitely felt it looked worse than Reach, at least on first impression.
The SP was nice, but the majority flew over my head, and it was much easier on Legendary than Reach was. I might be shit, but I found Reach absolutely crushing/amazing in that I'd find myself at the end of the game, with like a power sword and half ammo on a rifle, and just trying to find a way to scrap through was intense. That really didn't happen in 4, particularly after I figured out that the Forerunner rifle + DMR meant everything died easier. Didn't really play much MP, found the unlock system and the killstreak-esque weapon drops to be... interesting? The game went from being really well balanced to just "I've played this game all day and night, so obviously that means I should win". BF3 was also a culprit of that with the flying vehicles, but that's another game entirely.
So yeah, Halo 4 was really disappointing. The XBONE isn't currently even on my radar, but for the sake of all these fans and in the scenario I do get around to getting one, I really hope they fix this up with 5.