Reasons why:
1. Storyline did not follow the Halo Novel "Fall of Reach." Cortana was never delivered as a "package" but she was with the chief before and after the covenant invaded. This is just one example in the story that they decided to magically change.
2. Spartants look like dwarves, buffed up with no comparable height considering they are more than 7 feet tall.
3. Nade damage radius is crazy. Rocket launcher damage radius is also crazy. Overall weapon balance sucks and pales in comparison to Halo 3.
4. No competitive ranking system
5. Horrible multiplayer maps, which they thought it was somehow cool to incorporate from campaign. (Its multiplayer, not campaign genius)
6. Bloom! - For a spartan with a high powered armor suit and quick reflexes, it still amazes me how a weapon would shake in a "bloom" effect when fired considering he can kill a man with a single punch.
7. Theater mode is just for single player. It was fun in Halo 3 to bring your friends to watch a video in theater and look at the highlights of the previous game together. Can't do that in Reach.
8. No Bungie Favorites. Come on, you knew it was awesome to see the new pics, maps, and game-types each week in Bungie faves.
9. Can't see people's Fileshare. It was great pleasure before a game in the lobby to go through a players fileshare and look at any pictures he would have. Guess what, can't do that anymore. Because of this, Fileshare became unused in Reach. In Halo 3 this small event made Fileshare interactive, but had to kill it in Reach didn't you.
10. Jumping and nade bounce. To jump in Halo means everything. From getting to tactical places or for maneuvering shots, in Halo 2 to 3, jumping made the difference. In Halo Reach you can't jump over a pinecone. Also nades in 2 and 3 had more of a bounce, which made you use them with skill if you knew how to threw them and where. The nades in Reach feel like oranges that bounce 1cm and stay in the ground.
11. Focus Rifle. Come on, any reason you removed a covenant sniper weapon and replace it with a piece of barfed copy of the sentinel beam? Next time, stop dreaming and come to the real world.
12. Armor abilities are a shame to Halo, and the way that Bungie implemented them in Reach is horrible. Come on...invisibility blocks your radar? Tell me hows that useful to a Spartan that plans to sneak without the enemy knowing anything if he's there or his position. Jetpack destroyed control areas, and Armor lock is a waste and another shame.
13. What happened to the nice Bungie, who gave us free maps and created or rendered a better version for us...I don't know - Cold Storage in Halo 3. Greedy donkeys.
There is probably more I can still say, but overall Halo Reach is a disappointment to the franchise and to the loyal community. My hope is that 343 will fix most of these issues.