OK. I'm pretty sure whatever I cook up is going to fall under your purview of "powergaming", even if it makes thorough sense. But let's consider a few variants:
Precisely, my friend. Anything that I don't like about 3.5 is "powergaming" according to the anti-3.x-ers. An 18 Int wizard with 8 just about everywhere else is already "powergaming".
With your standard point buy, I managed to sneak in a positive CON score and kept the stats above 8 on the regular Human wizard. That said, there's absolutely no reason for me to not max out my stats if I'm in a pointbuy game. Wizard has precisely one stat that matters for what he does. Do you want more randomization? Use the other methods that the book suggests, such as rolling for stats. Do you want people to excel at more than one thing? Encourage them by increasing the point buy to, say, 32. You will actually help them that way because Fighters, Paladins, Barbarians, Rogues etc. require way more attributes to be competent at their jobs than a Wizard. That's simple optimization. Everyone optimizes their character. That dude who realizes Power Attack is a better feat than Toughness? He's optimizing.
There's nothing wrong with optimization in and of itself. The problem is when it runs into other 3.5 issues, such as the developer intent being to reward system mastery by putting trap options into the system.
Starting off at a non-standard age? Powergaming. Headband of Int +6? Powergaming. Putting all your level up stat increases into Int? Powergaming.
The latter especially can't be in any light seen as "powergaming", since a heroic fantasy Wizard has literally no reason not to keep constantly increasing his brain capacity.
I'm also really, really not a fan of comparing any edition of D&D, especially 4th, to Diablo 2 or WoW or whatever. I see this line of thought constantly perpetuated mainly because D&D 4E actually codified the roles of each character in their settings; roles that everyone already knew, more or less. Fighters were pretty much traditionally always Defenders with a dash of Control and Striker, Rogues' combat application was always Striker (buy lotsa d6es, time to get stabbing), Leaders are a good synonym for Healbot since it concisely presents the character and allows for some flavorful variants like the Warlord, and Controller is also a good moniker for many of the purposes of the Wizard. I honestly think the 4th edition is a very fun game, particularly for light beer & pretzels games which are mostly oriented around tactics and pushing minis around a battle map; everyone knows their role, you're not going to have any trap options, Fighters have a lot more interesting options on the battlefield than Press Full Attack. All the issue with lacking roleplaying mechanics in its core, I've always replicated by having the roleplaying mostly freeform with some d20 throwing whenever deemed necessary.
By that moniker, I'm not inclined to believe the 5th edition is "like Diablo 2" either. In fact, I've often heard that it's 3.5 that is more like Diablo 2, as some purist grognards used to comment on the WPL mechanic like this.
My DM wouldn't let me constantly run Mephisto for godly items to transfer for my level 1 Dread Necromancer for another campaign, for instance.