Totally agreed. But which is it?
The decision on any of those models, if not properly considered, could have impacts that reverberate through their product and userbase. Advertising, done wrong, could break their clean aesthetic. Affiliate marketing, done wrong, could hurt people's trust. You get the idea. The business model is a part of the design, in that it works in concert with it - and people are eager to see the fuller vision of posterous realized.
That said, I agree. I think it's great that they're taking some time to see how their product evolves, especially when they seem to sincerely want to "change the game." One thing YC companies seem to do well is that they have the patience to take the time to explore those markets alongside their users, instead of dragging them into a business plan that detracts from how users want to use the product in their daily lives.
EDIT: Whoops, this answers part of the question, from their FAQ:
Yes, Posterous is enthusiastically a free service. Later, we'll be adding premium features we know you'll love, but there will always be a useful free version you can use.
Sachin's mentioned planned premium features in the past, so I think that's a good bet. They could also do one-off deals with big companies, like they once did with Coca-cola and Loopt recently did with Virgin. They've got a good brand and smart founders, I expect they will do just fine.
Honestly, I startups often say "premium features" because that's something that users understand and aren't scared by.
Posterous has raise $5M, so they really need at least a $40M exit, preferably bigger. Are they really going to make enough for that kind of exit from a small percentage of their users paying, say, $5-10/month for premium blogging features? Doubtful.
I think their plan, as well as that of many other startups, is to grow as much as possible and then either find a way to monetize their large number of free users or get bought by a company that wants some combination of the users, the employees, and the technology. But the average user won't be reassured when seeing this in an FAQ:
"It's free, what's the catch?"
"We want to get as many users as possible while making no money, and then we'll find some way to either bring in revenue or sell the company"
Users would worry that Posterous either will die one day when they run out of money or eventually realize they need to make money and ruin the product. But "you'll always be able to keep your same free product, but we'll add some even better features that cost money" isn't scary.
This isn't unique to Posterous. I don't remember which startup there was, but I saw "premium features" on the FAQ of another startup and literally laughed out loud because I knew that the combination of the amount of funding they had raised and the small amount that most users would be willing to pay meant that that was a lie.
That said, I agree. I think it's great that they're taking some time to see how their product evolves, especially when they seem to sincerely want to "change the game." One thing YC companies seem to do well is that they have the patience to take the time to explore those markets alongside their users, instead of dragging them into a business plan that detracts from how users want to use the product in their daily lives.
EDIT: Whoops, this answers part of the question, from their FAQ: Yes, Posterous is enthusiastically a free service. Later, we'll be adding premium features we know you'll love, but there will always be a useful free version you can use.