(Note: I'm only looking at the freshman, sophomores, and juniors that were potential entrants to the NBA draft. We knew a long time ago that Jon Diebler and E'Twuan Moore, among many others, weren't going to be back in 2011-12; they were seniors. Also, this does not look at transfers such as Taran Buie.)
Illinois: Jereme Richmond (freshman) declares for NBA draft: I just about exhausted the Jereme Richmond talk here and here. For a team losing three starters plus an important role player to graduation, the loss of a freshman assumed to be inheriting major minutes next season is a serious blow to Illinois's rebuilding hopes. Richmond is a borderline 1st/2nd round pick; the latter is more likely.
Indiana: No plausible NBA draft picks; no one departed.
Iowa: No plausible NBA draft picks; no one departed.
Michigan: Darius Morris (sophomore) declares for NBA draft: Like Manny Harris the year before, Morris's departure came as something of a surprise; most expected both players to return after testing the NBA waters. The Wolverines go from expecting every player back from their surprising 2010-11 team to losing their most important piece. While this leaves a hole at the point for Michigan, and while Morris's departure will certainly hurt, there's still plenty of reason to be optimistic heading into next year.
Michigan State: No plausible NBA draft picks; no one departed.
Minnesota: Trevor Mbakwe (junior) and Ralph Sampson III (junior) forgo the NBA draft: Frankly, I wouldn't have ever included Sampson's name on this list if I had compiled it a couple of weeks ago, but Sampson looked to be gone on Sunday. Something changed, and athletic director Joel Maturi said he expected Sampson to return. The bigger return for Minnesota is Mbakwe, who was one of the dominant big men in the conference last season and one of the few bright spots for a disappointing Gophers team. Even with the transfer of Colton Iverson, Minnesota will once again be one of the tallest teams in the country.
Northwestern: John Shurna (junior) forgos the NBA draft: The only surprise was how long Shurna's name remained in the draft conversation. Unfortunately for Shurna, a quality outside shot alone isn't usually enough to earn a place on an NBA roster, but fortunately for the Wildcats, they will get one more year of Shurna's deadly outside shooting in Evanston. Returning almost everyone, Northwestern is once again hoping to break through to their first NCAA tournament appearance. Shurna's departure would have been crippling; now, that opportunity remains.
Ohio State: Jared Sullinger (freshman) and Deshaun Thomas (freshman) forgo the NBA draft: Thomas's return isn't too shocking; a quite talented freshman who was overshadowed by the talent around him, Thomas probably could have gotten picked in the draft at some point but can do quite a bit to improve his stock next season. Sullinger's return was a shock, despite his professions all year long that he would return. Normally, taking those sorts of claims seriously is folly, but Sullinger did not even go through the observation process. By returning, Sullinger passes up an almost-certain top 10 draft pick for another attempt at a conference and NCAA championship.
Penn State: No plausible NBA draft picks; no one departed.
Purdue: Robbie Hummel (redshirt junior) forgos NBA draft: Not a surprise in any respect, especially as Hummel recovers from his second knee injury in two years. There was never really any question as to whether Hummel would return.
Wisconsin: Jordan Taylor (junior) forgos NBA draft: Another non-surprise, although it was a little surprising that Taylor didn't take the opportunity to go through the observation process.