I think the main positions that the Colts need to draft are:
SS (or a DB who can be converted)
Offensive Line - Ah where to begin...The Colts need some serious truckers up front. The current line does fine in pass protection, but that's mainly because of Manning's quick release. Run blocking is much more aggressive than pass protection, which explains why the O - Line appears to be good at pass protection while they seem to struggle in run blocking. Some added youth on the line would either promote competition, or simply find replacements for the inadequate. The organization already knows this, and drafted Thomas in the 09 draft.
Prospectives: Russell Okung, Trent Williams, Mike Johnson
Linebacker - The Colts' linebackers are definitely an underrated crew and have been pretty solid this year (especially Gary Brackett), but as the injury to Hagler showed, they are not men of steel. Wheeler has filled in admirably, but more depth is required. The Colts also lack a stud pass rushing Linebacker (Session has 0.5, Bracket has 1 and Wheeler has 1). Coyer has thrown in more blitzes this year, but the sacks have still mainly gone to Freeney and Mathis. With the more aggressive style of defense being played, the Colts will have to draft players with more aggressive styles.
Prospectives (the Colts most likely won't get any of these, as plenty of teams will be drafting defense ahead of the Colts):
Brandon Spikes, Sean Witherspoon, Roland McClain, Travis Lewis
Fullback - As most all Colts fans know, the Colts have no true fullback. Gijon Robinson, Jacob Tamme and even Keyunta Dawson (correct me if I'm wrong, but I believe he was the backup DE who plays fullback) have played the position but lack the size and power of a true bruiser. With the addition of a fullback, the running game would surely improve. It would also provide a more reliable option in short yardage situations and spread the carries.
Prospectives - Toby Gerhart, Charles Scott
Strong Safety (or a big defensive back) - Yes Melvin Bullitt has been great in place of Bob Sanders (Pats game anyone?) but he can be injured too (and was in the Denver game). Since Sanders' status is always iffy, more depth at the safety position would be welcome.
Prospectives - Eric Berry, Taylor Mays, Joe Haden, Javier Arenas
Of course the positions (and prospects) are all personal opinion, but I think all of them would help. For each position I merely listed the "studs" who were entering the draft, but based on round placement, the Colts will most likely get only one of all the athletes I mentioned.