The most obvious and pressing needs are as follows:
Offensive Line
Again, don't get me wrong. This is still a pretty key issue. If we could draft a starting left tackle, and push Johnson inside to guard (his natural position) and get either Pollack, Devan, Thomas, or Richard at the opposite guard spot with Saturday in the middle and Diem on the right, the Colts line may be hard to beat. Don't forget this team was lowest in the league in sacks allowed.
Safety
Obviously the loss of Bob Sanders doesn't affect the team much, he has only played in 9 games the past 3 seasons. But losing him means we are no longer expecting him to be healthy, which means we will most likely play with Antoine Bethea and Melvin Bullitt. Now interestingly enough Bullitt's contract has expired, so the Colts will either have to sign him, or go with a rookie in the draft (I can't see Polian doing that). On top of all of that Jamie Silva is a free agent. I like Bullitt, I think he is a solid, fundamentally sound player, but I think the front office will likely see if they can get some value with a safety in the draft.
Wide RecieverThis is a position that seems strong at the time, but Reggie Wayne is entering the final year of his contract, and if we lose Reggie we go from having a "star-studded" receiving corps, to a receiving corps full of a bunch of #2 receivers with Pierre Garcon, Austin Collie, Anthony Gonzalez, and possibly Taj Smith. If the Colts don't want to fork over a large contract to an aging receiver after next year, they may draft one in the first round, and try to groom him into a number one receiver to take Wayne's place when/if he leaves. Plus keep in mind Garcon has had his issues (for a read at why I'm not sold in Pierre click here). And also we know the Collie and Gonzalez has had some issues with health.
Running Back
I know this may sound a little fishy. But the only running back who has a contract for next season is Donald Brown. And Brown essentially lost his spot to Addai and Rhodes in the final 3 games of the season. That means Addai, Rhodes, and Mike Hart are all free agents. I believe the Colts will sign one (or all of the 3). But because free agency is up in the air, and the draft is before any settlements of the labor disputes, the Colts may grab a RB as security.
Going forward I would like to look into these positions a little more in depth. The available college players and where the Colts may look for these players.