If it's a reputable, relevant store (and not MLM), a single sidebar link, and a real blog, I don't see a problem. The docs refer to Amazon specifically, but you can substitute another reputable store/site if there's something more appropriate. If you have a real blog about fishing, then we don't mind a referral link to your local fishing store that gets you a discount or credit or whatever.
Again, "real blog" is the key. WordPress.com is for blogs. If your blog is a real blog that real people actually want to read, the odd affiliate link is no big deal - worst case, if it attracts some complaints, we might ask you to tone it down a little.
The kinds of things the rules are intended to prevent are (including but not limited to) people who make many blogs, cookie cutter blogs with fake or stolen content, blogs plastered in blinking PTC banners, get rich quick schemes/scams, monetize your way to whiter teeth, guys in Elbonian internet cafes paid to create blog after blog after blog, and so on. Web pollution.
Writing rules that catch those guys while leaving everyone else in the clear is hard, probably impossible. So we have to be a little vague.
nb: clickbank, usercash, CJ and other pay-per-click services are so thoroughly drenched in spam juice that I'd advise against them even if your blog and usage falls within the rules, simply because you're likely to get caught in the crossfire.
I'll ask Mark to chime in and confirm what I've said, since he usually makes the final call on individual cases.