1. It takes a while to hide things (it also has a "hide too much, and un-hide later" mode, but that's barely an improvement).
2. The big one: it still downloads things. Let me be clear here: I don't want to have to wait for doubleclick to get off its ass and serve me an ad. I want the page all the way loaded when the bits I'm going to see are there. Which does not include adverts. I assume this is pending on Chrome adding a "pretty please may I?" hook to URL fetching - does anyone know when this will be here?
Putting ad-blocking inside the browser seems the most logical part, especially since you get the DOM already made so you know you look at the same data.
In all other browsers, they are basically a web OS. Stripping adverts out of web pages is well within a browser's remit. In particular, deciding what not to download is within their remit.