I think it lets Google off the hook too easy to say the algorithm is too complex to draw any simple rules from it. Yes, it's very complex, but any algorithm ultimately boils down to a bunch of if/then type logic gates. There may be a huge number of them, and they may be interdependent on each other in many ways, but it can still be reduced to simple tests and results.
As an analogy, consider a rules-based spam filter like Spamassassin. It's surely not as complex as Google's algorithm, but it has hundreds or thousands of rules against which each message is checked, some of them situationally dependent on each other. So it's true that you can't say, "Messages with these specific attributes will be blocked and messages with those won't," because there are too many different factors. But it is possible to say, "If your message is 90% URLs, there's a good chance it's going to get blocked. If your message uses the word "v1agra" numerous times, it's probably going to get blocked." The fact that's it's not possible to give simple, exacting rules doesn't mean you can't give general guidelines covering the clearer cases. And if they added a new rule that seriously down-graded messages with a particular attribute (or combination of attributes), they could say so.
In Google's case, I'm certain that their engineers have a tool that lets them input a page's URL and a keyword, and see a rundown of exactly how the algorithm treats that page and what "scores" it gives it for that search, both negative and positive. (If they don't have such a tool, they're stupid.) If they wanted to, they could make that info available through Webmaster Tools. Not necessarily in great detail, but they could let you inspect one of your URLs and report, "this page has these two major strikes against it."
I've always been a fan of Google, and I still use several of their products. (Reader and Chrome are too good not to use.) But they're making me tired. I've got a client whose site got hammered by Farmer. To be honest, this site was pretty much MFA at one time, but they've put a lot of work into adding content and making everything relevant over the past several years -- much of it on specific instructions from Google techs, including adding more AdSense when they recommended it. Now they're asking me how to fix it, and when I go to Google's support forum, all I get from their volunteers and interviews with engineers is, "It's not us, it's you. You're obviously doing something spammy/black hat. And if you don't know what it is, that just shows how corrupt you are that you can't see the difference between spam and quality." It's getting very old.