Brendan and I have been discussing bonuses to climbing, which has reminded me of something I said once about how I handle modifiers to rolls to avoid too much bonus inflation. It might be worth it to cover this as its own topic.
Let's take an attack roll as an example, instead of climbing. You sneak up behind a guard and try to wrestle him to the ground so you can tie him up. The guard isn't asleep or helpless, so presumably there's an attack roll. You toss a pebble in a different direction to distract him right as you strike. What kind of bonus do you get on the attack roll?
I think most GMs would figure it out this way: There's a +2 or +4 bonus to attacks from behind, plus your Strength bonus (wrestling,) plus your Int bonus (or maybe a Wis bonus) for the pebble trick, minus the guard's Strength and Int bonuses.
The way I do it is: the guard is alert, but isn't all that remarkable. Let's say you are stronger than the guard, but not as clever. Those two factors cancel each other out, plus you have the advantage of striking from behind and tossing the pebble; total bonus is +1. If you had been stronger and smarter, that's significantly more advantages in your favor, so it would be +2. I only look at who has the overall advantage and give a +1 to that side, +2 if there are several more advantages.
Here's another situation: you're trying to cross a narrow bridge without falling. The bridge is slippery and rickety, but you have high Dex. That's a -1; if there were also bees attacking you and you had an injured leg, it might be a -2.
So: having lots of factors in your favor doesn't improve your chance of success much, but it does improve your chances of getting a +1 or +2. This seems natural and easy to me; I could just eyeball the situational factors and think "oh, sounds like the PC has the advantage here." I don't discuss it much because I think it doesn't feel natural to most other people; they would rather have explicit additions and subtractions, and want some situations to be much more beneficial than others, such as rear attacks being easier than flank attacks, which are in turn easier than normal attacks.
There's only a few things that I count separately from situational factors, or that give bonuses higher than +2. Armor, for example, comes in various grades, and the number of hit dice a creature has adds directly to the attack roll. It's possible to treat these as just another situational factor, especially if you are modifying a 1d6 roll and don't want to give too high a bonus. For example, instead of giving a 9th level thief a +5 when trying to pick a pocket, you could give a +1 when trying to pick the pocket of a 4th level Fighter, or a +2 when picking the pocket of a 2nd level fighter; the thief could get a +1 when trying to pick a complicate lock while drunk with improvised picks. For 1d6 situation rolls, I sort of do this, in that any level bonus of +2 means no roll is necessary, anyways. But I still allow open-ended level bonuses on general principal. Those who would rather cap the bonus are welcome to do so, however, and the situational factor method might be one way to do so.