Sadly, I think one of the features now called balance is an equal ability to affect the game at any given time. This will never happen in AD&D, except perhaps by a million to 1 fluke. This is how 4E is "balanced" though, a sort of imposed equalitarianism upon any who play the game. Nothing you do will improve your ability to affect a situation, so levels, items, knowledge, and almost everything else largely become meaningless.
That's so not true.
4E certainly reduces the variance in how much you can affect the situation: in combat, everyone is always useful. (Out of combat, it varies more). However, they are useful in different ways, and there are times when you really need one person's abilities more than another.
In AD&D, you would have combats where the magic-user or cleric could basically end it all with one spell. (Sleep being the classic low-level option). Against that, you'd have combats where the magic-user would be very limited in usefulness (no spells, or spells that just weren't worth casting).
In 4E, a character is very unlikely to be using "I win" effects or be in a "I suck" position. However, depending on the situation, certain characters are more effective than others. When you're being attacked by swarms of minions, the ability of the wizard to cast Scorching Burst and kill many each turn is far superior than that of the rogue whose extra damage against one target is wasted. Then, against the solo or elite creatures, the rogue comes into his own.
Out of combat, things are less clear. I think it is safe to say that the 4e skill system allows each character to have clear areas of competence, and areas where they're not particularly useful. Thus, if there is a challenge that requires an athletics check, then some characters will be able to do it easily and others won't. Rituals also change the parameters, but do all characters have access to all rituals? Of course they don't!
So, the assertion that every character can contribute equally to out-of-combat challenges is also false.
The skill challenge system (which is not used for every out-of-combat challenge) is intended to allow a wider variety of characters to participate, but it would be still false to say that every character can participate equally. There are times when your character just doesn't have any applicable skill; there are times when you can blitz it with your particular skill set.
With regard to player challenges as opposed to character challenges, there even is a section in the Dungeon Master's Guide about them (more than in previous DMGs). Yes, it also offers advice as to making them character challenges, but this is not mandatory: it really does depend on your group.