I agree with a lot of what has been said in here, but OL and DL are a bit different than most other positions when talking about translating from colledge to the NFL. Now some schools run pro style offenses or pro style defenses that make the transitions much easier, but those players are also usually drafted higher.
All skill positions, apart from QB and by a smaller margin TE, are by far the easiest to translate. If a WR is fast, has good hands, and runs good routes, all he really has to do is learn the offense and work on perfecting his game. If a RB has good vision, ball security, can pick up the blitz, catch, and run routes, the same can be said for him. QBs have the issue of coming from offenses that don't translate well, the speed of the game, and the possibility of substandard mechanics. TEs' biggest issue is in blocking, as all NFL defensive players are usually bigger, stronger, and faster than what the TEs were used to in colledge.
The next tier in difficulty would be positions like Saftey and LB, although I would probably put TE in here too. The fundamentals of the positions haven't changed. Tackling, shedding blocks, coverage, etc, remain pretty much unchanged. Safties and LBers have to adjust to the speed of the players and the game, learn defenses that are almost always more complex, and develop skills that may have been secondary or unimportant to them in their college days. Most Safties and LBers run defenses in college so they know how to get plays put in and sorted out. Some LBers are converted from college DEs and have to learn how to drop in coverage, or some Safties convert from strong saftey to free saftey and they too have new nuances to learn. Either way the bottom line is that this group usually has skill sets that are incomplete, and not only do they have to develop these skill sets, they have to develop them to the point to where they are competetive in the NFL.
The last and most difficult tier would be the previously mentioned QB, plus CB, DL, and OL. I'll start with DL. It has also been said by many coaches, analysts, players, and others, that the difference in the level of competition between the college level and the pro level is the greatest at QB, CB, OL, and DL. I am inclined to agree with this.
DL get moved around. A 6'3, 298lb player who might have been a very good college 4-3 DT, might have to switch over to a 4-3 DE, a 3-4 DE, or a 4-3 DT with a different technique and responsibilities. This changes the offensive keys they have to read and react to. This also changes how they react, and most importantly, how they play. Things for these players that for the past four years have been coached and drilled into muscle memory, are now incorrect and need to be changed immediately. They have to make this adjustment and make it well in the first or second season or they are done usually. Like every other position, they also have to be able to make the adjustmant to the level of competition. This requires some to put on weight or slim down. Yet they all have to get stronger and faster. They also have to learn skills that a lot of college DLmen do not possess. Nearly every NFL defense uses zone blitzes that drop lineman into coverage. Many college defenses do not.
CBs who don't come from pro style defenses have a very high learning curve. Some schools don't do much man coverage, some don't do much zone, and very very few do both really well. So rookie CBs have to learn and develop both of these skills to pro levels, and I am inclined to believe that covering a receiver who is almost always bigger and faster, for a long enough period of time to where the rushers can get to the QB or the QB checks down, has got to be the hardest single thing to do in the NFL. It has to be. Yet these corners are also expected to learn how to blitz, react and help out on the run while not biting on fakes, and many other skills that are often neglegted in the college game. They too have to adjust to the difference in the level of competition. Covering a pro team's number 2 or 3 WR like Austin Collie or Jordy Nelson for 4 quarters is a great deal more difficult than covering some Sophmore number 2 WR regardless of the conference.
And at last the OL. Sure like all of the other positions a lot remains the same. Fundamentals are fundamentals. Yet OL have to learn new fundamentals depending on the offense of the team that drafts them. They also HAVE to get stronger and faster. They usually have to get bigger as well. Yet once again the glaring difference is in the level of competition. Gabe Carimi is a stud prospect. He was almost always lights out dominant for the Badgers. Yet that still doesn't mean squat in the end really. Even after a great training camp, perfect response to coaching, and great strides in the gym, I guarantee that he would still have trouble with Julius Peppers. I know that is slightly unfair, but their is a reason why good NFL DEs get paid in FA. Nearly all starting NFL DEs give rookie tackles fits, especially early in the season. Many OLmen can never make the transition to the pro game. Just like quarterbacks. Yet the difference is that a team only needs one quarterback to start the games.