Monte actually had very little to say about 4e other than that he didn't like its licensing. However, you can guess what he doesn't like about it from his comments on 3e, 3.5, and game design in general.
Practically Mandatory Miniatures
Monte on 3.5 (from here):
The game has an even stronger focus on miniatures. 3.0 had a strong focus on miniatures, but we wanted to at least address the fact that you might not want to play the game that way. But everyone in the Wizards of the Coast offices does, and so now you have to as well. And Wizards has a new line of miniatures to sell you. Seriously, though, for those wanting to play the game sitting on the couch, the game now offers a new barrier for you. The Combat chapter in the Player's Handbook now reads like a miniatures game. More and more of the game stats use "squares" rather than feet (or both). This is a huge step backward toward the "inches" used in 1st Edition.
No Vancian Casting
I don't have a link or quote handy, but Monte has made it clear on several occasions that he's a big fan of Vancian casting (or at least considers it a defining feature of D&D compared to other products), that is wizards having only daily spells, no encounter or at-will spells. Which is probably why when he was one of the design leads for D&DNext he brought it back (as discussed in some of the Q&A sessions transcribed here).
General Design Philosophy
Also, from Monte's blog, you can see that he's a fan of simulationism (3/3.5 were simulationist, 4e was narrativist/gamist) and that he prefers a more loosely written ruleset (i.e. one that encourages DM adjudication; the difference between 3.5 and 4e in this area is very much open to debate, but most 3.5 fans perceived 4e as having much less player improvisation / DM adjudication).
4e also butchered a lot of other sacred cows, and in general went in a very different direction from 3e, for which Monte was the design lead.