Page blocking, that's enough or a reason when it's used improperly.
When you do document.write, as Andy said, it's synchronous meaning you have to wait on it before continuing with the rest of the page.
I don't want your site to hang up just because your ad server is down. Unfortunately, this is the case with SO, the ADs on the right use document.write using a script from another server that, if down, blocks the page from loading until it times out. This delay, because of document.write and a slow (often third party) ad server is far too often the reason a site is slow to load.
Side rant: Advertisements gets you money from views, ok good, you need to make a living. But don't be dependent on them from a technical perspective, as in their server is down, your site is FUBAR...do ads in a non-blocking way, there are many ways besides document.write to achieve this.
