Cousins will be Al Jefferson part deux, at the worst. Physically, he certainly has some issues, between what might be potential conditioning issues and his lack of leaping ability (27" vert) and overall speed, but he has a condor's wingspan, decent enough lateral quickness and decent shotblocking instincts, so defensively he shouldn't be bad--in fact I project him to be average in the NBA here, and he can play both PF and C effectively. Offensively we've already covered it, and suffice to say he'll be elite in this level. Can't see how his pre-draft measurements will drop him out of the top 4, but it confirms my suspicions that his peak will be very good star rather than all-out superstar.
But let me put it this way: I wouldn't blame him if he passed on Cousins for someone like...say, Derrick Favors. There's a reason the Spurs want him, and as a preacher of physical tools, I agree: he's much more physically appealing. Offensively he's definitely not as skilled as Cousins, but he definitely imposes his athleticism--he's finishes well around the rim, gobbles up tons of offensive rebounds (and above average rebounding ability) and will deflect very well on both ends (in particular a very good shotblocker). And he's fit to continue this in the NBA, as his physical tools put him over the top--he has extremely good leaping ability, a great base with good strength at 6'10" 245, very good conditioning, and elite end to end speed (watch out for him in transition). He also has a contour's wingspan and can play center at spurts, and it doesn't hurt he's the youngest player in this draft class. There's much more upside with Favors than Cousins by virtue of his athleticism, and while there are definitely some flaws (turnover prone, black hole?, little range, doesn't get to the line all that well) he's not unlike Amare Stoudemire in many respects, and as a secondary elite player to a primary elite player, or to a very good PG who can run with him, he's built to thrive in the NBA.