I was going to mention is ability to hold onto the ball, but then I compared his stats against the rest of the league, and he is #13 in INT percentage with a 2.2% (10th if you remove all the QBs who started fewer than 10 games, but 12th among projected 2014 Starters (Aaron Rodgers, and Josh Mckown started 9 and 5 games respectively, you could probably throw McKown out of consideration here, but that only bumps him up one spot). Sure he's only thrown 19 INTs in his 2 seasons and only 9 this year, but he also doesn't pass as much so he is throwing the ball to the opposition at a greater rate than a lot of the top QBs who have more INTs.
So compared to the best QBs he is no more efficient at not turning the ball over. He is sacked a lot, and gives up a lot of yards compared to his contemporaries on said sacks (272 yards on 44 sacks). Out of the 38 QBs who were sacked last year his sack percentage of 9.8% (roughly one sack per 10 plays), ranked 35th.
So the few things that I was under the impression separated him from other QBs turned out to not be true. I think he could eventually be a 93 overall QB, but right now he isn't. His mistakes aren't as costly as they are for other teams because the defense is so stout and he has a strong running game to cut down on the number of times that he is asked to make a big play with his arm.
Strangely enough I have no issue with Romo being where he is ratings wise as long as his confidence is easily shaken in the game. Romo is asked to make up for his team's lack of defense far too often. Anybody remember the Broncos game last year when Romo was practically perfect until the final drive, Anyone remember how the defense crumbled making a game winning drive necessary, Romo makes one mistake and everyone forgets that the defense blew that game by having more holes than swiss cheese. Same goes for the Packers game, though that one can be pinned on him more because he should have lead his receiver on the Shields pick, or he could have not audibled out of the running play and the Cowboys could have run the clock to just about zero.
Romo isn't clutch, and if you rattle him he's gonna show it with his play, but when he is on he is one of the better QBs in the league.