Now, I have a lot of ideas about why they want that, and also about why they feel they're not getting the satisfaction they want from attempting to Do Things. For the time being, though, I'm going to wave past that. Instead, I'm going to propose what in some cases might prove to be a band-aid, or in some cases might actually end up adding a fun extra to the festivities. I think the best way to impart this might be an example:
Bernaise: Can I try to shield rush this guy in these crusty old rules from the 80s?
Bernaise: Whatever man, I want to charge little-doghead-boy and Knock Him Down baby!
Troll: Sure, you can try practically anything. Just make a regular attack roll.
Bernaise: But what if I've practiced shield rushing things? What if I'm like.. Mr. Shield Rush?
Troll: Well, you haven't yet, so go ahead and give it a try. I think it would be stretching believability quite a bit to think medieval-style training would specialize to that fine a degree, but everything starts somewhere :)
Bernaise: If I do it all the time, is there any way I can get better at it?
Troll: A +1 bonus for anything is a pretty big deal in this game. A longsword does 1d6 damage, but that enormous zweihander does 1d6+1. And that little-doghead-boy has even less hit points than that.
Bernaise: It doesn't make any sense though, that if I'm constantly trying to trip people or shield rush them or jump over pits that I'd never get any better at it.
Troll: It does if your guy doesn't have any ability in that.. as I recall, you practiced and practiced and practiced to learn how to sing, and you still sound like sh..
Bernaise: Yeah, yeah, I know... but my character is a fighter, and this is a fighting thing.
Troll. Okay.. make your regular roll to hit. Every time you try to shield rush, whether it's a success or not, YOU make a checkmark or something, and keep track of it. Then let me know what the total is the next time you make a level.
Bernaise: Sweet!!! I'm successfully limiting my options to excel in an easily expressed specialty!! Woot!!And there was much rejoicing. I think it can be seen to make a moderate amount of sense, and it shouldn't terribly unbalance anything. I think I'd probably do something like subtract his strength from 100 and want to see that many attempts at level-up time to give him a +1 shield rush, or some other effect, maybe a stunned opponent, a morale check or execute a 10 foot withdrawal, or various other possibilities.
Granted, I'd rather handle these things on the fly. I think they work better that way, and then both the DM and the Player can come up with something unique that really fits each specific situation. Of course, that means the DM has to take on the responsibility of making each fight or trap or parley a unique and specific situation, even just in how it's described. I think if a player is still having trouble expressing on-the-spot detail, but they do want to try, this can be a pretty harmless semi-solution. It's still completely specialised, and case-by-case, which I like.
Let me know if you can think of similar things a character might try learning, and how you might go about deciding whether they've learned it or not.