Please vote on the poll and elaborate on your thoughts in the thread.
"No 3rd party content" means a return to the "Bad Old Days" where TSR made D&D adventures (aka modules), and others did only at their extreme peril. Frequently going so far as to not stat monsters, or make similar sounding monsters with similar stats. Sometimes in a similar sounding system.
So you wouldn't be able to buy a Dungeon Crawl Classic from Goodman Games, or an Adventure Path from Piazo (even if they wanted to make them), nor from Necro.
That would be very sad, and I'm not likely to buy 4e if it turns out to be the case. By "not likely" I mean "Can I get my pre-order money back? Store credit would be fine." And "I'm unlikely to buy it later either, I remember the dark days, I know how that movie ends, I didn't like it last time, and I'm not going to like it this time either."
You also couldn't buy a set of new monsters (which sounds like fun!), or magic items, or spells, or whatever. I don't really do this very often, and would miss it more in the abstract then in reality. That sort of restriction would make me think twice about buying 4e, but wouldn't be an absolute bar.
This would also make me sad. I might cancel my pre-order. In fact I'm likely to do so, but may or may not buy it in 6 to 8 months depending on how things work out.
If it is "just" that you can't break apart the game and rebuild it from the ground up and make something like Monte Cook's Arcana Unearthed, then that's not such a good thing, but I never bought those books anyway. It would make me a little sad that such freedom is gone from the world, but it won't slow down my pre-order.