It comes down to mentality and style of play, for me.
I don't want my center with any fu-fu frills. I don't want my center shooting threes. I want my center to be a freeking panzer tank. Someone who moves around the court at a decent speed with little to no attention paid to life/limb. I want my center to be a workhouse who will rebound at an elite rate, catch the ball and keep it high, and dunk on any sucker who happens to be in his way. I want someone who will anchor our defense and cause offenses to settle for jumpshots because he's gotten in their heads after blocking 5 shots in a quarter.
Screw finesse. Screw shots past 12 feet. Give me the hooks, the dunks, the power moves, crisp passing, and deft hands. I want my center to be a blunt instrument but clever as hell when it comes to putting the ball in the hoop if he catches the ball within 10 feet from the hoop.
Basically, I see Ellenson as a finesse player and possible KO replacement. I'm more attracted to the Diamond Stones, Domantas Sabonises, Jakob Poeltls of this draft. I wanted Willie Caulley-Stein last draft. I grew up with Perkins on the Celtics, and Jake Voskuhl/Hilton Armstrong on the Huskies.
I want my center to be, above all, big, mean, strong, and active. I think the Celtics would really benefit from having a center like that, rather than, you know, Tyler freeking Zeller.