I'll chime in to say that one of my biggest beefs with 4e at the moment is with the skill system.
While I like the basic structure of it, with pared down skill lists (I know this is not everyone's cup of tea, but IMO it's one of the things 4e got 'right.') It's far, far too easy to jack your skill bonus in some skills to ridiculous levels; too many untyped bonuses to skills.
On the plus side, it makes it simple to get 'good' at things, especially for inexperienced players, but this has two bad side effects.
First, you can easily have a *huge* gulf between skill levels, even between two characters that are supposed to be good at the same skill. This isn't necessarily bad in and of itself, but when player A wants to be the party lockpick, and so makes a modest investment in it, only to have Player B come along taking every single bonus to Thievery they can find... well, it's just frustrating for Player A.
Secondly, and perhaps more sinister, is that there are too many ways to 'substitute' one skill for another. Many of these effects are per-encounter, with the Dailies reserved for Initiative substitutions. This becomes a really big problem when you have, for example, Player A the lockpick specialist with his modest investment beaten routinely by Player B using a skill power or similar to just substitute a different skill that they've maxed out for both lockpicking and also whatever else it does.
Player A feels ripped off, and outshined - and I wouldn't blame them one bit - but Player B isn't cheating, so what can you do?
One of the biggest offenders for this is the combination of Backgrounds, Themes, and some of the newer Essentials class feature skill boosts. All untyped bonuses. Probably the worst offender for this is the Arcana skill and the Order Adept theme. The worst part being that it's trivially easy for an Arcanist to dominate skill challenges by subbing their ridiculous Arcana check for other skills (particularly social skills) on a per-encounter basis.
One of the parties I'm involved in has one such character in it, and at level 10 he has a perfectly legal Arcana check of 24, which once per encounter he can sub (once each) for Diplomacy and Intimidate, and then either one again or Bluff with Arcane Mutterings. So, by investing pretty heavily in one skill, and a smallish investment in powers (two cantrips and a feat-purchaseable skill power), he can do all the heavy lifting of the party face as well as that of his 'skill role.'
This character can trivially make at-level hard skill DCs, and can hit ones 10 levels higher 50% of the time. There is something fundamentally wrong with that. It's really hard to plan and balance for.
And he could have gotten his skill a couple points higher than that still had he chosen an Arcana-bonus race, a different background, started with 20 Int, and taken Skill Focus instead of Bardic Knowledge. But he was trying to 'diversify' a bit. He's also got respectable scores in Nature, Religion (better by far than the Cleric), Stealth (hard DCs on a 6+), and several others.
I know there are ways to deal with this, but they're all houserules at this point. It also can lead to whiny players who don't understand why you're targeting them and their level of system mastery when they're "only playing by the rules." Stuff like slapping a -5 penalty on skills used in this way may sound ok on the surface, but not only does it penalize the players with a more median skillset, it also further encourages the min/maxer to squeeze every last bonus point to make up for the "unfair penalties."
So tl;dr version - 5e (or whatever) needs to patch the holes in an otherwise streamlined skill system.