Your argument is an Appeal to Authority. "Smart people invested, so it must be a good thing." Social proof is useful for making certain types of decisions, however discussions about posts on HN are not decision-making exercises, they are--ideally--analyses of the arguments advanced in the post itself.
If the analysis is wrong, we ought to be able to dispute it directly rather than assume that since all that big money is in the game, they must have analyzed these arguments and they must have disputed them.
Lots of other people are digging into the arguments themselves. But it's not just about the superior analysis of the investors, it's about their superior access to information. The idea that two simple, broadly available pieces of information (investors taking money out and completely transparent, though non-traditional accounting practices) are obviously disqualifying and yet people with strong track records are investing huge amounts of money makes me suspicious.
There are other signals of silliness here, most conspicuously the comparison to Madoff and Ponzi Schemes. It's link bait; even if Groupon is being manipulative with their accounting, they're still providing a real service to real people. They're providing value to businesses and customers. If their argument was about the sustainability of the model, or even just about the justification behind recent valuations, that would be something I'd be more interested in approaching in the way you describe.
I guess I wasn't aware that HN's limitations on ideal discourse are so severe. I figured there might be a way to add value by getting some perspective about the implications of this argument and their improbability.
You're making one very big assumption, which is that professional and well informed investors would not want any part in it if they knew it wasn't a sustainable business model.
They might know better than we do when this thing is going to blow up. Or they might have an idea for a transition to a more sustainable business model once Groupon has reached ubiquity.
Right now it looks to me like they have misjudged how quickly opinions would turn against Groupon's business model or rather against their accounting practices. An SEC investigation isn't something any investor, no matter how professional and knowledgeable, wants or expects to see.
There are no limits on actual HN discourse, which is why you are free to argue that the presence of big money signals that something, somewhere must be right about this deal. Likewise, there are people using this post to rehash critical arguments about Groupon that have nothing to do with this specific post.
Ideal discourse is exactly that. As they say, "In theory, theory is the same as practice. In practice, it isn't."
Those "powerful investors" are just fools looking to sell to greater fools. Of course they didn't realize that no, this actually isn't 1999 and you can't just dupe the public into buying your overpriced IPO again. Rich people make mistakes too, all the time, and lose LOTS of money doing it. Never forget that.
If the analysis is wrong, we ought to be able to dispute it directly rather than assume that since all that big money is in the game, they must have analyzed these arguments and they must have disputed them.