1. This new "feature" is basically automated filters. Most folks who've used gmail for years have at least some filters set up. Google didn't say anything about throwing ads in the promotions tab, so this is a little...strange.
2. Slippery slope. How long until this populates the main inbox tab? I bet google could even charge differently depending on which tab you want your ads to show up in.
3. XMPP Federation. Google Reader. Google Wave. How long will it be until Gmail is either axed or full of so many ads that it's unusable?
4. Does Adblock / NoScript block these? Can it?
5. The history of Gmail was that it was a next-generation email service and started as an invite only club. Now, every email provider under the sun gives similar features, and gmail hasn't changed much. Personally I think that this is a good thing, it shows that the service is stable and the ideas were correct. But people at Google might think otherwise and might want to try to "innovate" more. Sooo... ads.