Last season, the Miami Dolphins started a rookie quarterback to open the season for the first time in franchise history. Ryan Tannehill, drafted 8th overall out of Texas A&M, had started only 19 games in college, and was about to start for a professional football team. The scouting report on Tannehill coming out of college was that he had terrific physical tools, could make all the throws, had above average mobility, but was inexperienced and raw. He would need time to develop before being able to start a game. But the Dolphins went against conventional wisdom and let him learn on the field, in a trial by fire.
His statistics for 2012 paint a picture of the typical rookie QB, mediocre numbers that could certainly use some improving. Tannehill finished the 2012 season completing 58.3% of his passes for 3294 yards, 12 TDs and 13 INTs. His QB rating was 76.1, putting him at 31st out of 38 QBs that had significant playing time in 2012. Tannehill was right behind fellow rookie Andrew Luck in QB rating and two spots head of another rookie, Brandon Weeden.
The consensus is that Tannehill had limited weapons to throw to and didn't get the best pass protection. Outside of Brian Hartline and Davone Bess, who had 1 touchdown a piece, Tannehill didn't have too many targets. The third wide receiver was a carousel that included cast-offs such as Ledegu Nanee, Jabbar Gaffney and Anthony Armstrong. Marlon Moore was more of a special teams player and Rishard Matthews was a rookie taken in the 7th round.
At tight end, the Dolphins had Anthony Fasano, a good blocker with good hands, but lacking the sufficient speed, quickness and athleticism to beat good linebackers one on one. Behind Fasano, the Dolphins had Charles Clay, a 2nd year player who had an up and down (mostly down) season.
Out of the backfield, the Dolphins had Reggie Bush, Daniel Thomas and Lamar Miller. Bush was underutilized as a pass catcher, but throwing to running backs is not how quarterbacks win games in the NFL.
The Dolphins also had an offensive line that wasn't very athletic, so they were ineffective at running screens, which would have certainly helped the running backs be more effective pass catching threats out of the backfield.
All in all, Ryan Tannehill was saddled with an offense that wasn't very dynamic or versatile. On top of that, he had his own flaws, tied to his inexperience. Tannehill would at times stare down receivers, read plays late, take too many chances downfield when hitting an open receiver short for a first down would have been enough, and even struggled with accuracy on deep throws at times.
The point is, there was plenty of room for improvement, both in the talent surrounding Tannehill and in the quality of Tannehill's play as well.
However, despite a low quarterback rating, a completion percentage below 60%, and a low number of touchdowns. There are things Tannehill did very well. Below you will see a statistical breakdown, provided by Pro Football Focus, that will paint a clear picture of what Tannehill did well and what he didn't do so well. You'll also have an idea of where improvement is needed in surrounding talent from looking at some of these metrics.
Rating By Week:
First, let's look at Tannehill's rating by week according to PFF. What you'll note is a player who had his ups and downs (shocking for a rookie). He had some very good stretches with some tough weeks in between. But for the most part, he was in the positive range on a week to week basis.

Passes By Direction:
I think this is one of the most telling diagrams regarding Tannehill's performance. In looking at this chart, you see where Tannehill's strengths and weaknesses are from a throwing perspective, but we can also add some flavor regarding where the Dolphins have some offensive weaknesses as well.

For example, when throwing deep down the left sideline, Tannehill went 3 of 18. A big reason for this may be that in many of those scenarios, Tannehill was throwing the ball deep to Davone Bess, who lined up on the left side of the offense most of the season. Bess's strength is as a slot receiver, not a deep threat. Additionally, other players that may have been involved in these deep throws were the cast offs we talked about earlier. Miami's best receiver in 2012, Brian Hartline, lined up almost exclusively on the right side of the offense.
Conversely, Tannehill had a lot of success throwing deep down the right sideline to Brian Hartline primarily. When throwing deep down the right, Tannehill was 13 of 26.
Tannehill also struggled throwing the ball down the middle of the field. 7 of his 13 interceptions came when throwing the ball on short or intermediate routes down the middle.
This indicates three things:
1. Tannehill needs to work on his decision making between the numbers
2. The Dolphins need at upgrade at tight end
3. The middle of the field is clogged with defenders because teams don't have to worry about the Dolphins attacking deep
Passing Under Pressure
Tannehill's numbers when under pressure also show something interesting. In certain cases, they actually improve. For example, his yards per attempt when under pressure are higher than when he is not being pressured. His yards per attempt against the blitz is a remarkable 8.1. Of course, when he is not under pressure, his completion percentage goes up over 60% and his QB rating is at its highest. The moral of the story is that the Dolphins have to do a better job of protecting Tannehill, but he is mobile and composed enough to make plays even if he is under pressure.

Miscellaneous
One other factor that dampened Tannehill's numbers is drops. Tannehill had 36 of his passes dropped, 11th most in the NFL.
Conversely, one area in which Tannehill excelled was at running. Tannehill ran the ball more as the season went on, and it made him a more dynamic and dangerous player. Granted, you do not want your QB running the ball too often, or being tackled for that matter. But if he can create that extra threat or extend the play with his legs, it will help his receivers out quite a bit as the defense will have one more thing to consider.