In the past, we had a nice canned answer for MSO posts regarding junk questions and answers:
All new posts, retags and edits come with a free bump to the front page, so it's easy for people to see spam, flames and noise.
Now that the main page displays user-specific posts, does this need to be revisited? My thinking is that each individual bump now gets onto fewer front pages, so it's easier for bad stuff to slip by.
Then again, that doesn't necessarily translate into fewer eyeballs. After all, people probably weren't checking on bumps for tags they didn't care about even before the change. Although I feel like SO has been slightly spammier lately, I have no hard numbers. It could be natural random variation, or coincidence affecting posts I've happened to see recently.