I guess where I struggle with the whole roster construction issue is that we went to a final four a year earlier with a similar roster: NBA caliber PG, spotty 2G, the same three guys at C, the same guy at the SF and quality (though obviously very different) PF in Southerland. I think you could even argue that very few would take southerland over grant at the 4. I suppose Triche was a more versatile and better 2, but I don't know that I'd point that out as the difference, particularly since the MCW/Triche combo was just miserable shooting the ball.
So, while I see where everyone is coming from on the roster construction angle, I'd suggest it's a combination of three factors (IMHO):
1) Tempo
Already covered this to a degree but I just want to make it clear that we didn't have to play like Loyola Marymount but not strongly encouraging Ennis and MCW to push the ball in unsettled situations (which I'd have to assume was a conscious decision, though I have no inside info on that) seems really strange to me. Especially with as dangerous as MCW was in an unsettled situation and how pedestrian he was (until the tourney) in the halfcourt.
2) Post touches/development
These probably should be separate points but I'll combine them b/c I'm lazy. It's a chicken-egg scenario but we haven't consistently gotten the ball to the post since 10-11 and we also have had essentially zero threat to score or create from there in that span. That's bad. We need to have more inside-out to this offense AND we need the staff and the bigs to step to the plate to actually offer some sort of incentive to pass the ball there. To be fair to the staff, Melo probably had that ability and was headed that way before *ahem* departing, and Grant had the potential to be such an unusual and dangerous player next year with the physical tools to dominate down low, out-quick guys going to the basket from the win/high post, and potential to keep teams pretty honest with an improved jumper. Regardless, the fact that DC has been such a colossal disaster (yes I realize he's had injuries), Rak still doesn't have a post move anyone could call remotely close to a go-to basic move, and none of the wing guys (Fair, KJ ...) has developed any sort of back-to-the-basket game has hurt this program's offense the past few years.
3. Guys jumping to the NBA/moving on
This obviously is part of life in college sports and college hoops in particular. But, though there is no one to blame, per se, the attrition has really hurt this team. It's one thing when you lose a super freak like Wes Johnson, MCW or Waiters with eligibility left or a really polished kid who had a phenomenal year decides to cash in (Ennis), but losing kids who still have so much room to improve (Melo, Grant) in addition to quality seniors like Scoop, Fair, RJ, Triche (to some extent), Southerland, is a really tough balancing act for coaches. It puts programs in a spot where something as stupid as a random NCAA clearinghouse issue for a month or so potentially has a major impact on your season. If Roberson is playing with the team in Canada and then available for big minutes vs. cupcakes early, he would have been a valuable contributor late. It hurts that he wasn't b/c it was pretty clear that the lack of depth was a difficult issue for this team.