Out of all the problems I have with Reach, the only really severe one for me is the map selection. It just isn't good enough.
It's somewhat concerning I think, because map selection has grown increasingly weaker throughout the games. Of course, it was far from perfect in H1/H2, but both had a pretty strong selection. H3 had some good ones, more bad ones, and the rest were pretty mediocre. Reach's... well, let's just say it's easily the weakest set of launch maps in the series.
It doesn't matter how good the gameplay is, if the map selection isn't there to back it up.
The good maps in Reach aren't as versatile as the ones from earlier games either, which makes the entire situation worse. Let's take Lockout, for instance. You can play good games of Slayer, KoTH, Oddball, even 1 Flag and 1 Bomb work pretty well. The map might come up a lot but the experience still feels fresh because of the difference in gametypes.
Compare this to say, Countdown. Slayer is the only good gametype on there as far as I'm concerned. CTF/Assault is ridiculous. KoTH... eh maybe.
In short, there are dramatically fewer good gametype/map combinations, probably an order of magnitude below H2s. And that's why the experience - for me at least - has got tired so quickly.