Reach's failure to deliver on a campaign level wasnt beacuse it was dark...dark stories are fine, better if they are in the context of a series where the sad ending of the dark story helps lead to ultimate victory.
My problem with Reach's campaign was:
1) The characterizations of Noble team were dry, uninteresting and humorless. Being serous isn't neccasarily a bad thing but the characters of Reach were just boring. No interesting interplay of dialogue, funny interesting or otherwise. No interesting backstories or motivations. No real dynamic-ness to them either. I'm not someone who demands character arcs, and I don't expect a masterpiece of story telling but if Bungie could do it with previous Halo's, they could do it with Reach. I could hardly care when they died.
2) Lack of Scale, sense of the struggle. Reach rarely delivers a sense of absolutely gigantic forces clashing. Only in a few scenes do you see more than one covenant or UNSC ship at a time, whereas in Halo 2 and 3, skyboxes and cut scenes full of clashing fleets were a common occurence. The arrival of the massive mainstay of the covenant fleet following Long Night of Solace's destruction feels like a rushed aside, and we do not see the collected force in any large numbers. The cutscene at the beginning of H2 gives more of a feeling of desperation and destruction to Reach's fall than anything in Halo:Reach actually does. Also, Long Night of Solace is one of the coolest things in the campaign, but we see it in action only once and it is completely destroyed within one mission.
3) Unsatisfying conclusion: The final Lone wolf Mission is an interesting capstone but the pillar of autumn mission does not cut it. It feels like the same grind through Covenant infantry interspersed with a few moments of Fire-fight esque hold the fort action. The brief appearance of the Scarabs (who you never fight in any meaningfull sense) feels shoehorned in, and the last few minutes is simply a little bit more firefight.