I don't use WP.com for blogging, but I read a lot of sites on here and there is an intermittent (but very frequent) problem with the posts and pages loading slowly or not at all. The titles of the items will appear in the tabs, but nothing else will happen - I will just see a blank white page. Sometimes, I will eventually get an unthemed, single-column page with the text all in Times New Roman in black and white.
It doesn't happen if I load the page in a mobile browser (which triggers WPTouch), only if I use a normal browser. I also found that the Stats were very slow, such that they made my self-hosted WP blog (including admin) extremely slow until I disabled them. I also found this forum slow loading at first.
This has been happening several times daily for several months, but judging by past forum topics (all now closed), it's been going on for years. A DDOS attack was mentioned as a possible cause in an earlier thread, but it seems that the theme server and stats are affected but not the server that holds the content.