[Icelander]

Personally, I have always found the idea that all modifiers were supposed to be worth the same in all circumstances profoundly unintuitive and contrary to real-world experience.

It seems obvious to me that a skilled person doing something he's fairly confident of accomplishing can afford a slight handicap much more than a novice whose chances of accomlishing the same thing are touch and go. As an example, a minor quirk in the controls of a vehicle, enough for a -1 to skill, will rarely cause an experienced driver to crash during a challenging tight turn he's done before more times than he can remember. A novice with a fifty-fifty chance of making it under perfect circumstances, however, will be much less able to afford a minor handicap in that situation.

By the same token, a large bonus will be much more meaningful to the odds of a low-skill individual accomplishing a given task than for a high skill one.

d20 systems don't reflect this reality.

Nor do they reflect the massive disparity in the odds of success between a master and a novice, in general. The lowest possible odds are 5% and the best are 95%, but that's not the whole story. One problem is how competent unskilled people are, with a complete amateur having a 50% chance of success with something fairly, but not oveerwhelmingly challenging.

Considering that an average small-town professional (skill+ability modifier maybe +4) has maybe a 70% chance of success at a moderately challenging task and a world-renowned master has a 95% chance (skill+ability mod +18 or more), this is too little difference. Imagine that a world-class football (what the Americans are pleased to call 'soccer') player was only scoring twice as often in a penalty-kich shoot-out with a regular Joe.

Compare that to GURPS, where a novice would have only about a 5% chance of success at something moderately challenging, while a professional with skill 12 does indeed have something close to the 70% success chance above, but the odds for the master reach 98% at only skill 16 and increases past that allow the highly skilled character to retain that success rate under increasingly adverse circumstances or when faced with increasing challenges. The football player now only needs skill 14 to overwhelm the regular Joe, scoring roughly ten times as often.

Since we all know that it doesn't take world-class skill to succeed twice as often as someone who doesn't have much of an idea of how to perform some task, we all have an intuitive sense that things ought to work in a way much more akin to the bell-curve than the d2 linear probability curve. Or at least we should.