A major problem of his is that of getting strength on his headers. When was the last time you saw Carlos thump a header all the way back out to midfield? Laursen and Mellberg to an extent did that magnificently, but Cuellar seems unable to head the ball further than 10 yards. He gets no height and distance on his headers at all.
Playing the 4-5-1 helps alleviate this weakness as there are more midfield players to pick up the ball after Carlos has headed it out, but in a 2 man midfield as seen in the Wigan and the Rapid game, Carlos's weak heading only gives the opposition another chance to attack.
Also, he gets turned rather easily, and loses players who have a tiny ounce of movement.(see Rapid game)
Another minor criticism I have of him would be that he likes passing square, but let's face it, we've had this lack of passing confidence problem for a long time now.
His strengths though are that he takes up brilliant defensive positions, and seems to possess the uncanny ability to pre-empt where the shot is going to end up and gets himself there before the shot is even struck. And some of his last-ditch tackling is superb.
I think he does have some good qualities, but he needs the proper partner to perform the weaker areas of his game for him. Cuellar will never be good enough to perform the Mellberg-role(closing down opposition players running at the defence) because he doesn't have enough pace and gets turned easily. With a proper "Mellberg" beside him however , Cuellar could then just focus on sweeping up at the back, where his ability to take up good defensive positions and his great last ditch tackling can be utilized to it's fullest.
Davies was supposed to be the "Mellberg" for Cuellar, but it just hasn't worked out as of yet, part of the reason might be Curtis's shoulder problem, I don't know. But I would hope that Dunne's arrival will help bring out the best in Cuellar.
I still think when fit, Davies is still the much more promising defender.