I've worked for companies where one client has an inordinate amount of influence over the company, why... because they are the biggest client, they represent a significant percentage of the company's revenue. Market forces inevitably can create situations where a particular client is "really good" for a company, they may pay more than other companies would, etc and, if this client is lost (or even just reduces the business they give the company) serious consequences can result including laying people off, etc. Of course Wikipedia can avoid this, at least initially, by not relying on one company, going through an ad-broker, etc but its possible that at some point a situation can arise where they are faced with a dilemma (and this would probably be years into the future after an infusion of money has inevitably re-shaped the organization to some degree) where a decision has to be made to either lose a client (advertising customer) or bend to pressure from them to censor or delete something... I'm just speculating here but past experiences of mine lead me to believe something like this could occur as companies bend to pressure from their clients all the time.

With all the dollars at play, I reckon there could be unforeseen ways to game the system. Maybe some "Enron" type will scrutinize every wikipedia bureaucratic procedure and term and condition of how the ad sales works and run some arbitarge keyword games that would distort the content in an ultimately un-usefull-to-the-user but profitable-for-the-gamer way.

I also worry about the increased tracking on politically sensitive pages or controversial topics.