I agree, Horford should be in the conversation. After a slow start to the season he's been a remarkable luxury to have off the bench. Shooting 42% from 3 this season, 44% post All-Star break.
I think the only knock is how many games he's missed (which of course were intentional for rest). I think a lot of people will disqualify him for 6 MOTY on this alone... Actually, now that I look at it, he still played 65 games this season. Should be in the convo.