So do "powergamers" not exist in 4e? You don't need to worry about making sure a 4e adventure is challenging to every PC of the same level?
Powergamers exist. But the "average" PC is a lot easier to define due to a lack of extreme customization. At level 11, I know that the bonuses to hit are going to range (vs AC) between +13 and +18(lower than 13 is possible if you completely ignore the PHB's advice on how to make a character, +19 or +20 might be possible with some extreme powergaming). I can safely put the enemy's AC at 26 and know that PCs will have between a 40% and 65%(between 30% and 75% if we extend it to the absolute limits) chance to hit. Which means a reasonable chance for anyone. Plus, the way the game scales is that the difference between lowest and highest should stay close to the same for all 30 levels(it grows by 15%).
The difference between best and worst is small enough that I don't have to PLAN for the powergamers, they just have a slightly easier time of it. The lower end of the scale can purposefully flank or have leaders to give them bonuses to overcome the small disadvantage they have as well.
Contrast that to 3.5e where the minimum at 11th level is the 6 strength wizard who decides to use his non-magical staff to attack(+3) vs the 26 Strength Half-Orc Barbarian who is raging, has Reckless Rage, Weapon Focus, and a +3 weapon, (+27) means that an AC 27 creature can be hit between 5% and 95% of the time. The highest misses only on a 1, the lowest hits only on a 20. The difference between weakest and strongest gets further and further apart the higher level you get in 3.5e. At 20th level it is possible to have one character have 4 attacks with a 95% chance to hit while the lowest hits only 5% of the time with ALL his attacks.
Wouldn't the way to go about making an adventure be for the writer to assume the PCs are "normal" PCs built with the "average" builds provided from the core books? Then leave it up to the DMs to adjust things depending how many optional books & rules he has allowed in the game? I can't imagine creating adventures with the goal in mind to make it balanced for every type of PC...that would be a nightmare. It's the DMs choice to use more options. If he chooses to go down that route, then it should be his responsibility to make adjustments.
We were writing for Living Greyhawk. The worldwide campaign where the DMs didn't have a choice about what kind of characters to allow. Nearly everything was allowed(with a small list of 30 or so banned items due to them being very overpowered). 28 point buy. DMs are not allowed to modify the stats of the creatures, the adventure must be run as written to avoid favoritism or "unfair" games(i.e. getting a bunch of XP for defeating hard monsters when your DM lowered the difficulty on them or having your party wiped out because the DM didn't like you).
The thing is, I'm not even sure what to assume average IS in 3.5e. If you consider the 16 strength 11th level Rogue with a +1 weapon and no feats to add to his attack bonus. That's +12. It hits on a 16 what the above Barbarian hits on a 2. If the Rogue is "average" and you create an adventure for that(AC 26 enemies to make a 55% hit chance) then the powergamers can walk through the adventure without any challenge whatsoever(the barbarian hits with his SECOND attack 85% of the time).
Now if this isn't even an issue in 4e, that's good. But is it not an issue because there aren't as many options yet? Could a writer like yourself be in the same boat as you were during 3e several years from now when there are more character options for 4e? Or are the rules setup where this just can't happen and players are unable to powergame now?
The stacking rules have been set up in such a way that almost nothing stacks with each other. That helps. The authors of 4e have realized this exact math problem(they've posted about it a couple of times) and have purposefully avoiding putting in any more feats or items that give you any more bonuses to hit.
In 3e, they never knew about this...so they didn't avoid breaking the game. I currently have my fingers crossed that they stick to their guns and avoid publishing anything else that stacks.