Terrence Ross has a prototypical frame for a 2-guard at 6'6" 195. While he's a little slim, adding weight to Ross' frame and weighing him down isn't something NBA teams should be anxious to do.
His athleticism allowed him to do just about anything he wants on the offensive end at the college level. He has the quickness off the dribble to get by his man cleanly and get to the hoop where he can finish above the rim. He was a serious matchup problem for smaller guards at the college level, capable of going down low and posting up to get buckets. While he's not Hakeem on the block, Ross uses his height and lift on his turnaround jumper to get clean looks at the rim.
His explosiveness lets him get to the rim pretty much at will on college defenders. Unlike his teammate Tony Wroten, Ross creates separation when he gets by his man; once he beats his man, they will not recover. When he's not getting clean by his man, Ross has a suddenness he uses to create separation for a jump-shot with a silky and consistent release. He's smooth and consistent shooting off the dribble going left or right, and his range extends to the NBA 3 point line. When he gets open looks from three, he will knock them down.
In terms of skill, Ross has a refined game as he usually knows what he's doing on the offensive end. He occasionally gets rushed, but towards the end of his sophomore season his decision-making seemed to improve and he developed a greater feel for when to assert himself, which was probably the most notable and important aspect of his growth as a player. While that growth may have been stunted during the early part of the year as he adjusted to Wroten's ball dominating ways, the fact he was able to evolve his game and make an impact on the offensive end nonetheless speaks volumes about his understanding of how to play (otherwise referred to generically as "basketball IQ").
Ross isn't a pure play-maker for his teammates, but he doesn't have tunnel vision either. When attacking with the dribble drive, he's able to locate teammates when the help comes. He looks to score, but doesn't force too many shots.
Defensively, he still has some work to do. He moves well laterally and has the foot quickness to stay with pretty much anyone at the college level. He seems to lose focus and get caught off-guard from time to time, but it would be wrong to characterize it as a lack of effort -- Ross plays hard, eagerly chasing the fast break after a steal looking for clean up baskets. He plays the passing lanes well and has the burst to intercept telegraphed/lazy passes. On the defensive end he simply needs to anticipate better in on-ball defensive situations to fulfill his potential on the defensive end.
With his physical gifts and skill-set, Terrence Ross should be a lottery pick, and has maybe the highest ceiling of any off-guard prospect in the draft. Based on the way he plays and limited film study, Ross appears to be a willing worker who will put in the effort to maximize his potential. He doesn't hold the ball for extended periods and is a good spot up shooter (though that isn't really his game) so he would be able to slide into the Mavs' rotation immediately.
His greatest immediate value to the team would be his ability to get into the paint and finish -- something the Mavericks desperately need. If he is around at 17, as many of the mock drafts floating around the interwebs project, this is a no-brainer.
Best case: Kobe Bryant
Worst case: Quincy Lewis