Nothing! In fact, the new issue attachments code uses plain ol' Markdown to show the image. The reason we added this is because arcane Markdown code is impossibly hostile for new users and beginners.
Have you ever had a manager/user/whatever person issue a screenshot of an issue using markdown? It doesn't work. Github is supposed to be for teams too (private repo's), and this brings it 1 step closer, but still far from what other services offer for issue tracking.
1) Hard to use for mere mortal users (QA, testers, normal users posting issue for an App, etc. 2) If uploaded to another site, the images are under a different protection domain (for protected repositories.)
Seemed like the perfect balance between, hard enough that you don't throw everything on there and eat up space, but useable enough that it works fine when you need it.