I've never seen Cooper play, but right off the first concern is size.
Now size is not everything, but if we are looking at the draft, teams tend to stay big. For example the first Dman taken in 2010 that was under 6'2 was the 7th dman taken, last of the first round. Average size of Dmen in the first 6 picks was 6'3, average weight just over 200 pounds. 2009 saw 13 dmen drafted first round (must be near a record), only one under 6 feet tall (Ryan Ellis). 2008? 12 D men taken, only one under 6 foot was Karrlsson.
What that means is if you are not 6 feet or more, and want to be a first round pick as a Dman, you better be more than exceptional at the level you are playing at.
Now Cooper is a late Bday so he has a bit more room to grow, but 5'9 175 (got that size off hte blog posted here, is it correct?) really means he has to shine well above the other players to get a nod as the top dman.
Even Ebert whom I consider only a tad small, is probably heading to the draft at 6'1, maybe 6'2 and 200 pounds.