I'm a huge WCS guy so I'll try to give the obsession a voice
1. Great shotblocking
2. Pick and Roll defense: He is a fluid enough athlete to guard 3 of the 5 positions. He has the potential to switch onto guards in the pick and roll and smother them. Considering we have no bigs that can even stop dribble penetration on the pick and roll switch, this would help us.
3. Rolling Hard to the Rim:His ability to roll hard to the rim and catch lobs (he has very good hands), means that help defenders need to play in the paint to bump him before he rolls otherwise he's got an easy alley oop. This will improve the quality of shots for his teammates. Similarly skilled big men like Chandler and Jordan lead their teams in offensive rating and it's because having an athletic rim running center frees up shots for others.
4. Potential: he is tall and fast. There are rumors that he has a decent jumper that he hasn't been able to show off at Kentucky. You can not teach size or athleticism, which means that players with both have huge upside, WCS has both (he was fluid enough of an athlete to be an all state receiver in HS, how many centers do you know with the quickness to do that?)
5. Need: Everything WCS does well is a weakness for the Celtics
6. Culture: In extending Bradley, drafting Smart and (to a much lesser extent) trading for Crowder, Ainge has added young players who are very good defenders. Adding WCS gives the C's another player who is a defense first guy to grow with.
It's hard to say. Would prime Kendrick Perkins or Omar Asik project to be a top-10 pick? I think that's the caliber of player than WCS projects to be.
I think comparing the athleticism of Perkins or Asik to Cauley-Stein is not accurate. While WCS isn't guaranteed to produce like either of those two, if you are comparing him to others you have to go with more fluid defensive centers like Chandler or DeAndre.
To me the best comparison is DeAndre Jordan with a serviceable free throw shot and maybe even a servicable jump shot. I think a player like that is absolutely worthy of a top 10 pick.
One scout said Cauley-Stein is appealing because defending the pick-and-roll is critical in that league and he can do it in a number of ways. He can also legitimately lock down a small forward, power forward or center at the next level, the scout said. He has defended point guards at times in college.
http://www.courier-journal.com/story/sports/college/kentucky/2014/12/16/kentuckys-willie-cauley-stein-nba-draft-stock-quickly-rising/20505973/