I think it had to do with initial accessability compared to older Halo's. I got into 3 late, and 1 and 2 I only got access to at friends homes, due to my not having an OG Xbox.
What did you like about it compared to the others?
Everyone on HaloGAF appreciated Reach's clear feedback system (I liked how clear it was but me and other still wanted melee bleed through) hitscan, godly netcode... that's about it. Those were pretty universal likes. Then it started getting messy.
I wasn't great at Reach, but I felt decent/good enough that I could play without going negative for the most part, where in past Halo's I definitely struggled. Customization options were great for Spartan's, and I'm big on customization in games. Once I got it later on the map packs helped in a lot of ways with more enjoyable maps to play on.
There's definitely shit I hated on, as can be shown with previous posts I've made in Reach's thread.
The story also wasn't great, but I found it more enjoyable than other Halo's. I did like firefight with friends. I did like daily challenges.
Plenty to like in the game, and plenty to build up on for going to H4.