My biggest gripe with the ICON system was the dice system as well.
Maybe there was a knack I never got around to get, but on too many occasions, I felt that, as a GM, my options were limited to : "OK, either I set the difficulty of this task to 9, and the player has a 90% probability of success, or I set it to 10, and the player has exactly 1 chance out of 6 to succeed".
This is especially annoying when you want some tasks to be challenging, and you discover that you only have a one or two points margin between ridiculously easy and nigh impossible.
My other gripe was the fact that some characters could become almost unbeatable in combat. For instance, a player once created a Klingon character whom we calculated could single handledly defeat the other three characters, even with the help of phasers (as he could withstand a full phaser shot which would have vaporized any other player).
My final complaint was that I felt there was too narrow a margin in skill numbers, meaning that the difference between a beginner, a professional, and an expert was usually one or two skill points.
Otherwise, the system was rather nice to play. I especially liked the renown and the combat system (minus the part about very strong characters).