Spontaneous thoughts in no particular order:
It seems to me that either a supporter tier (i.e. a small(ish) amount of money per month with not even any particular features attached; maybe a badge or a stylized highlight of the user name) or regularly happening donation runs (like Wikipedia does) could be successful without annoying anyone. Plenty of other websites or content creators do those and are just fine, financially. I think many people are aware these days that the internet is not free, and are willing to pay a bit for sites that they deem important.
Generally, ad "partners" will always be in a tug-of-war against the rules, no matter how strict those are, and will always, by the nature of the business, try to go right to the edge of what's allowed; being as annoying as possible, while just skirting a ban. This sounds like so much obvious trouble to me; I don't feel SE is in a position to invite this right now?
Once ad "partners" have big payments going towards SE, they will be able and willing to influence the content of the site. Maybe obviously not in the first months of the feature, but possibly down the line. And not necessarily openly, but indirectly, by threatening to pull their business if certain things are not changed by SE.
Heavy users will just come up with ad filtering solutions right away and nothing much will come from it? Casuals or really new people will quickly be turned away? I know for sure that when I see a new site that has frequent ads, especially intermingled with the regular content, there is little that turns me away quicker.