Not really any change. I just checked, and from about a month of spam, there were maybe 2 messages (semi-junk from web sites that I wouldn't really miss) out of 110. I usually do check (though not necessarily empty) the spam more frequently.

I am not sure of the details of Google's spam ranking algorithm. The opposite thing: what if spammers marked everything in their spam box as "not spam" ? Actually, I don't think spam needs to do this. It already is using an extremely efficient natural-selection tool: the human brain.

You could just be a victim of a slight variation in the changing spam entropy, as some type of message similar to your emails becomes more spammy. Without a tool to show the actual scores of messages, asking here is probably the best you can do. (If there was a tool, it would only encourage gaming the system more.)

Does Google's algorithm somehow take into account how often you delete something from your inbox?

I am still subscribed to my old company's newsletter, and before, I used to like staying updated with whatever was going on there. Now every time I get a newsletter I instinctively just trash it without opening it.

Lately, I noticed that the newsletters are going straight to the spam box.

Nope.

I use priority inbox, though, which seems to permanently whitelist email from an address marked "important".

I'm not sure - there has been a 3-5x increase in the number of spam emails in the Spam filter for me since around the beginning of the year. As a result I just don't look as intently anymore.

I caught a SoundCloud email in the Spam the other day and rescued it.

I have. Some of my team has been sending me emails and I have not been receiving them. First Amazon Cloud, then PS3 Network, then LastPass,now Gmail... is our world falling apart?

I had an email from Google(docs) go to the gmail spam folder.

fwiw, I've always found google's spam filters too sensitive for my tastes. I use a pay service I like (spamstopshere.com)