I think Reach definitely has a huge amount of potential. Certain aspects of the sandbox have really impressed me, particularly how each weapon now has it's own niche, which is more reminiscent of H1.
With regards to reticle bloom, I'd rather see that removed for precision weapons and just have the rate of fire locked slower. Bloom is great for rapid fire weapons like the AR because it provides feedback on just how inaccurate your shots are. It's like health in H3, this was there in previous games, just not directly visible. Making it visible allows the player to make more informed choices.
But for precision weapons it just introduces RNG, and that's extremely frustrating. Being nailed in the head by people firing 2x as fast as you due to luck does not feel satisfying, it feels like being cheated. Statistically yes, it happens less than you should win if you're taking controlled shots, but it leaves a very bad taste when it does happen.
With regards to the base player traits (speed and jump height), I think that's a huge problem. The jump height results in getting stuck over what feels like trivial obstacles, which breaks the flow of the map (getting to the grenade launcher on Powerhouse is the one I have in mind here). The player speed (sans sprint) makes it feel like a long time before you can get around, which slows things down.
I'd say part of the problem is that the maps seem to expect you to have jetpacks if you want to get the same sense of movement you could get in earlier games. Two of my favourite maps, Midship and Lockout were partly as good as they were because there were so many ways to get around. If you had required a jetpack to get around them anywhere near as freely, then they would have played horribly. Okay, Bungie don't want the jetpack to me almost useless, but you can still have it open up new ways to get around without greatly restricting things for those on foot.
The other main issue I think is Armour Lock. It's not the concept I find bad, just that at the moment it acts like a giant reset button (plus blocking melee attacks whilst instantly draining shields... don't get me started). Previously to reset a fight it would take good coordination to pull off effectively and reliably. If it were tweaked to give you a limited opportunity to block attacks with good timing, instead of a spammable reset button then I reckon that would help a lot.