1) Reach isn't a good game ,the story makes no sense and takes a nice steamy one over all the established canon that Bungie considered 'integral' to the game. Not to mention the fact that ripping apart all the backstory which they paid people to create and spent a lot of effort making seems like a silly thing to do.
2) Multiplayer is completely out of balance, hit detection has some serious problems (sometimes DMR kills in 3 shots to the head, other times it takes 10?) and the Scorpion/Banshee are overpowered to the point of ridiculousness. Every time I use either my kills go into the 40-50 range and its gets pretty silly
3) There are barely any maps, what do we have for big team battle? Paradiso, Haemorrage, Spire (godawful map) and Boneyard (godawful map, one Scorpion - srsly?) and the closer battle map which really isn't designed for big battles. Since three of the maps are ripped from campaign they aren't balanced for online and don't have the fun gameplay of Haemorrage
4) DMR needs damage degredation at range, from what I've seen it doesn't appear to have it. What's resulted is getting damaged from ridiculous distances and the battles get boring
5) Gameplay is dull, it doesn't have the extra skill of Halo (Health was a bit more important, headshots to shields did more damage than a bodyshot to shield) and the 'abilities' make very little distance. Massive lack of anti-vehicle weapons so the heavy vehicles dominate the game, Warthog now sucks and is basically a ticklecannon that takes two people to run. Also the new vehicle damage is silly (they have a set amount of hitpoints and then explode when they are used up - my scorpion has just randomly exploded from one DMR shot before when my HP is at full)