The largest issues for me are:

1. Gimicky game elements that lack depth
2. Uninspired level design
3. Lack of new challenge
4. Spartan Ops

Large sections of the game are segments like flying the death star trench run, boosting the ghost through the earthquake, mowing down stuff with the mantis, piloting the pelican and quicktime events. The problem for me with these is that the scenarios are rather simple, with no real room to play or improve or really scale with difficulty. You have a few easy things to get the hang of, then you can do the segment easily. This doesn't really inspire me to repeat those parts of the game.

The level design for me again did not really take advantage of any of the new things offered in the game. I did not see any level that stood out from anything in any of the other Halo games besides the Gimicky stuff listed above. Marathon infinity used the same engine as Marathon 2, yet the levels design made it feel totally new and fresh. Same with the fan maps like Evil, Tempus Irae, etc. Halo 4 used a completely new engine! Why are all the levels still the same? Why aren't we seeing Infinity / Evil / Tempus Irae style innovation in our levels? Add to the fact that many of the stages were deliberate throwbacks to previous games, the experience really was more of the same. That's not bad in and of itself, but…

Lack of new challenge. You can take all your skills from Reach and prior and finish Halo 4. There simply aren't any new techniques to master. The new enemies are killed the same way as the old ones. The new weapons work the same way as the old ones. Rather than playing it and realizing how much I have left to learn, I play it and realize how much I already know. Can I beat RC Master's times? Not right now. But why would I want to when I can be working on a new game that requires me to learn new stuff?

I might be trying the new spartan ops, but I have no HD space so I have no way to play them.

I do not think the Halo formula is stale; I think the design and imagination of the games is stale.