here is my opinion which is more or less shared by the community.

positional star rating is important for all positions

qb- passing, intel and vision are the big three (in the order of importance). agility used to avoid sacks/pressure according to the admins.

Olineman- str and blocking equally important. most of the community says footwork helps pass blocking. it probably does. however, it appears to me that agility actually has a bigger effect. (look for a min of like 16 str, preferably 18+). at lower levels where you run the ball, all 5 olineman are important. at higher levels, always play your best OLineman at OT.

WR- catching, catching, catching. you can pretty much stick anyone with high catching and he will do well. Strength appears to improve tackle breaking and MAYBE speed might help. agility probably also helps too. speed is not as important here in GI as it is in real life.

TE- exact same as wr except early on, blocking to help you block. str is probably more important here if you use him to block for the run.

RB- speed doesnt seem to matter as much as str. carrying to lower fumbles this position diminishes its importance as you get higher in competition. at higher levels of competition, the RB (hb or fb) is used mainly as a extra lineman because they help blocking.

on defense
Dline- Lots of str for DT. tackling early on to avoid missed tackles but later on, tackling wont matter for DT. agility probably helps
for DE, lots of str, good amount of speed and agility. agility seems to have a big importance in getting pressure and sacks.

OLB- speed and str is important. both should probably be around 15+. tackling is the most important. positioning seems to help pass coverage and int. agility seems to help both pass coverage and tackling. Intelligence probably helps.

MLB- same as OLB except str is probably more valuable. Intelligence is VERY important. they make the decisions on defense. tackling is also very important. all things that apply to OLB also apply to MLB.

Safeties- free and strong safety pretty much the same.
speed is probably important, str probably helps. Intelligence is also very important. again, all things that apply to LB applies here.

cornerback- tackling and positioning are important. agility also helps. speed is inconclusive but will probably help.

You have read this, right?
http://www.grid-iron.org/index.php?page=community&subpage=viewt&t=4096