I think D&D does exactly what you're suggesting. You can get XP for anything - loot and killing is the basic way, but quest XP can be far more lucrative if the DM decides it should be. I think even Basic D&D says you should give XP for defeating monsters rather than killing them, and "defeating" can include by-passing them.
In D&D terms, at level 1 Merry has a great many skills that no-one knows about, because at that stage no-one has defined what they are yet. By level 20 he's realised he's good at all sorts of things. The thing is that in D&D they aren't pre-ordained: the player can decide what sort of things he's going to end up suddenly being good at each time he levels up. The only difference is that your system would allow skills to come into play as they're needed, rather than after the event.
All you need is a tweak rather like the one we brought in for last Game of Thrones( which I now find is pretty much how WHFRP does things): when people level up, give them points to spend on new skills which they can deploy as and when they need them. So in Pippin's case, it may be that he had a left-over feat to allocate from the last time he levelled up, and at the crucial moment with the Troll the player decided that it was Power Attack, plus maybe a few points into Discipline and Concentration and Knowledge:Troll Anatomy.
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In D&D terms, at level 1 Merry has a great many skills that no-one knows about, because at that stage no-one has defined what they are yet. By level 20 he's realised he's good at all sorts of things. The thing is that in D&D they aren't pre-ordained: the player can decide what sort of things he's going to end up suddenly being good at each time he levels up. The only difference is that your system would allow skills to come into play as they're needed, rather than after the event.
All you need is a tweak rather like the one we brought in for last Game of Thrones( which I now find is pretty much how WHFRP does things): when people level up, give them points to spend on new skills which they can deploy as and when they need them. So in Pippin's case, it may be that he had a left-over feat to allocate from the last time he levelled up, and at the crucial moment with the Troll the player decided that it was Power Attack, plus maybe a few points into Discipline and Concentration and Knowledge:Troll Anatomy.