The square would be way too big for the mobile experience in the forums, though you're right it could collapse to a 300x50 persistant forum unit. Not bad yes.
You definitely couldn't use the big ads like that though, as on a smaller monitor you'd run out of forum room super fast with the massive ads. Plus, we couldn't run background skins, and any time we did that it would have to be adaptive vs. static ad sizing. This makes it very hard to have the entire ad sales force out there hawking units, as each package would have to be done custom per client. Its very convenient for our site to fit within the specs laid out by GrindMedia, so that campaigns can run across all sites without too much changing.
I'm not saying that its a bad idea to go elastic width - and hey if we were starting from scratch I might even go for it. However, that change is way too drastic, and it would take so much time for us to do we'd have to skip out on a million other improvements that in my mind are more important.
Also you'd be super shocked at how much people hate drastic change. The best change you can do is always the most simple. Launching something like that would have everyone completely flip out... happens all the time. Hell once I switched the order of ski gabber and non-ski gabber in the 2nd tier nav, and there was practically angry mobs outside of the office.
Good concept, but go more subtle.
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