1st of all nope its not a common feature. I played many RPG games but I never find this feature.
Explain me this thing u said.. "If the deed is too easy you aren't learning anything new by it"
As if you ask me you are always learning from new things. If the thing is easy then may be you going to grasp it easily but that doesn't mean you are not going to learn anything from it.
Like.. If you don't know how to make a tea but you are a good chef still you going to learn how to make a tea if some one going to teach you.
When we speak about games we must take in account two aspects:
game balance and simulation.
The 1st aspect is quite easy to grasp: you need to balance the various features in the game to try to give the best experience to the gamer.
The 2nd one deserves a bit more explaination:
"simulation" means the game use some basic assumptions to represent the game world, the character development, etc.
When you use a XPs/Levels way to handle character development the general assumption i usually that a higher level character will be better than a lower level one. Of course the build is important but what I said it's true for identical characters.
This make leveling increasingly more difficult and the increased difficulty could be accomplished by increasing the number of XPs required for that, by reducing the XP gain, by reducing the perks gained by leveling or by a combination of these.
The XPs given for a certain task will usually be based (this is game balance) on the relative difficulty of the task compared to the level of the character.
For example some games stop to give you a significant number XPs if you kill low level enemies.
When speaking about quests the reasoning is the same: it all boils down to the relative difficulty of it.
TW3 has a very linear leveling system: you start needing 1k XPs to level then this number increase only slightly. I'm at level 26 and I need 2k or so XPs to level up.
Most of the times the quest of correct level give you a lot of XPs so you are often overleveled for some of them.
If the underleveled quests would give you the same number of XPs you would find that:
1) from a simulative POV, you will get a lot of XPs from trivial tasks, which is against the assumption made before
2) from a game balance POV, you will overlevel all the quests, so you won't find any fun in doing them, because they would be too easy.
I can concur this is not a perfect system but it's better than other ones.