http://www.escapistmagazine.com/art...the-New-Dungeons-Dragons-With-Designer-Mike.4 (just above the "next page" link)
"I'll use the magic as an example - we want the audience to be informed and know that if anyone does anything that lets you have two concentration spells at once they've broken the game. You can't have cloudkill and hold person at the same time. Now, in your home game, you can do whatever you want. We just want everyone to know how the game works and want everyone to know what's out there in the system and what we learned from playtests before we turn everything loose."
And ultra-buffing was certainly one of the things that made 3e casters broken, and a pain in the bleep to run. Classic examples would be fly and invisibility, or invisibility + summon monster (summoning monsters wasn't an attack in 3e, so you could just hide and pump them out). Or for that matter 3.0 haste + anything.
That entire question and answer:
Bolding: What's the kernel of an idea on what an OGL or licensing may look like?
Mearls: I don't want to go into too much detail because a lot of things are up in the air, but I will say that when 3rd Edition launched in 2000 there was this land rush mentality, and I think it makes sense from a business perspective. If you're a third party publisher you want to make sure you're the first to the market. Well, in their rush, you end up with people designing their adventure without the DMG. I was one of those guys. We want all the resources available, we want all the materials available, we want people to have been playing the game. We also want the audience to be informed.
I'll use the magic as an example - we want the audience to be informed and know that if anyone does anything that lets you have two concentration spells at once they've broken the game. You can't have cloudkill and hold person at the same time. Now, in your home game, you can do whatever you want. We just want everyone to know how the game works and want everyone to know what's out there in the system and what we learned from playtests before we turn everything loose.
It just seems to me that he is not talking about making sure that the third party vendors know how the game works before putting out an OGL, not that that two concentration spells up at the same time is unbalanced. I think he is using the term broken to mean that the third party vendor is breaking the core rules of the game by doing this, not that for someone in a home game to do so would be unbalanced.
It seems like you took this out of context because if he is actually talking about the game being broken with multiple concentration spells (as you suggest), then that would just be rambling on his part. Concentration has nothing to do with OGLs or to the question that he is answering. But vendors changing the rules of concentration and ignoring the OGL in that respect, does have something to do with the question he is answering.