A great difference between combat rolls and skill rolls is that skill rolls often are linked, whenever you make even the simplest plan.

Let's pick this simple example, something a skilled rogue would expect to do:

"I sneak past the guard, climb up the wall, and then listen at the window."

That's three skill rolls, and they are *linked*, as you obviously cannot attempt the next if you failed the previous.

Let's look at the probabilities, making all those three rolls equal for simplicity's sake:

* If you assign each skill roll a 50% chance of success, the entire plan has only a 0.5^3 chance of success, which is 12.5%...

* If you assign them 70% chance, the entire plan stands at 0.7^3, which is 34% - still a lousy shot.

* If you assign them 90% chance, the entire plan ends up at 72%, where you may start to consider doing it...

Looking at it from the reverse, if you want this rather simple plan to have a 90% of success, then the individual steps need to have a success chance of 0.9^1/3, which is 97%.

And that is a very simple plan, with just three rolls. Just adding another character would raise it to six rolls. At 70% chance of success for each roll, that would give the entire plan a chance of 0.7^6, i.e. 11%... At 95% chance of success, i.e. a 1 in 20 chance of failure, these two thieves together would succeed with this simple plan only in 74% of the cases.

(When comparing skill rolls and combat, just for a second imagine a melee system where you were "out" at first miss...)

Warning, anecdote:

I used to play in a campaign where the DM wanted things to be "challenging" for the players - so most things were difficult to succeed at. No matter how good you were, the difficulty simply seemed to rise in a magically matching way. And he kind of liked to, well, remind you of your failures again and again, whenever summarizing the evening's play, the campaign arc, or reminiscing about past adventures.

And after a while, he started complaining about what a passive and cowardly lot of players he was saddled with... nobody ever took any initiatives... nobody did anything without safeguards...

Now, happily, I play in [MENTION=2303]Starfox[/MENTION] campaign. Not only is his homebrew very good for skill-heavy play, with a normal-distribution dice rolls which means you can trust them most of the time, and limited re-rolls that smooths the worst swingy bumps without making it boring, but he is also, as a GM, good at encourage you to dare to do, well, *Heroic* stuff - and having a rollicking good time doing it....