I've given you tables of success probabilities by skill total and difficulty for two systems (World of Darkness, Shadowrun 4th ed.), plus a graph for Heavy Gear. Here I present that information again in graphs, plus two more systems, to show some of the different patterns that exist for success probabilities with increases in skill among different systems.
I really like how the Normal distribution of probabilities of success works in simulations, but not necessarily the way that GURPS implements it in the absence of difficulty levels. In real life, when we encounter tasks far below our skill level, we are quite likely to succeed at them and have a low variance with our high success rate. When we encounter tasks far above our skill level, we are quite likely to fail at them and have a low variance with our high failure rate. Tasks closer to our skill level have increasingly variant success rates. Because of this, I am in favor of the use of normal distributions of probability of success in simulation systems. This typically requires rolling more than one die and summing the results.