Great atmosphere, interesting choice of story and characters, but I feel it didn't go far enough with it, and also didn't have enough story.
Point 1:
The whole point of the ODSTs is that they are ordinary humans, yet the controls for them were just the same as SPARTANs. You could wield turrets, take down a million bad guys alone, and generally run, jump and dodge just as well as a super soldier. If they had scaled down the abilities and focused on a team based shooter from the POV of a normal human amongst a squad of other normal humans, there's a new interpretation of the series.
Point 2: ODST takes me several hours to complete, yet it doesn't feel like it does. Take Reach vs ODST. I could finish both in about the same amount of time, but I would feel like I got more from Reach. This is because Reach is epic in every sense of the word. You go from a routine patrol to watching an entire world burn as you heroically fend off an ultimately undefeatable wave of enemy troops. Compare and contrast to ODST where you heroically potter around New Mombasa looking for the rest of your squad and occasionally play a flashback mission. The game only got truly epic for the grand finale, but for Reach the same level of excitment was reached by the time you were fighting your way towards a space shuttle to take off and defeat the Covenant among the stars.
Don't get me wrong, I love ODST and I still play it. But I have never been interested by multi-player, and the single campaign left a lot to be desired for me.