I'll have to admit, he did better this year than what I thought he could do! I'm can't think of who to compare him to though, so I'll explain my opinion by comparing him to a bunch of receivers and I'm sure someone could come up with a comparable player.
Speed--- Santonio's best quality! I never doubted it. He's not the fastest ever, but he can beat most.
Size---- He's small at 5'11 and a little light too!
Separation--- This is the one thing that surprised me the most this year. He normally only got separation in college ONLY due to his speed, but he's learned a couple of skills in the NFL.
Closest comparison so far, Steve Smith, but that's the good stuff.
Ego--- Hopefully this can be drilled out of him, but he has a poor ego. He did a very smart thing mid season by simply shutting up, but the damage was done early. The best comparison is Keyshawn's rookie year. He thinks he is better than he is at this point.
Smarts--- I'll throw route running in here too. This is his biggest weakness, he's not just dumb, he's plumb dumb. He doesn't understand the importance of running a back side route and doesn't care. He doesn't run routes the best even when he's a primary receiver anyway. He'll take a play where he should be a safety valve and turn it into a play were the QB gets killed or throws the int. If the Steelers can turn this one thing around, he'll be a heck of an asset for the team. (Note alot of this part of my opinion has to do with what I saw as a Buckeye fan, but I've seen enough Steelers games this year to know he hasn't improved enough yet.) The best comparison is Kevin Johnson.
Work Ethic--- He doesn't have any, but that seems to be the trend with NFL WR's. He's a party animal off the field. He won't/can't block so all you non-Steelers fans thinking he can replace Ward as a WR should ask a Steeler fan about Ward's blocking ability. The best comparison here I can think of is Randy Moss this year, but Randy gave up trying to be a complete guy. I don't ever think Holmes will ever be a complete guy or will ever try to be one. Chris Carter had a huge influence on Moss, and I have a huge feeling that Holmes will look at Ward like he's speaking another langauge. Best comparison, any WR drafted by Detroit other than Roy Williams or Jerry Porter, take your pick
I would shy away from him big time next year due to the whole sophmore jinx thing, and I think his flaws will hurt his developement, but given how quietly he put up the numbers this year, you could get a bargain. Just buy at your own risk. I see a high ceiling and a low floor.