During breaks from college I worked at a wine and cheese store in St. Louis with a good selection of bottles under $10, and those were the ones I bought when I wanted to take something home (making $8.50 an hour probably also had something to do with it, but our wine experts did the same thing). I also sometimes got to taste much more expensive wine while I was working, and while some of it was outstanding, a lot of it didn't taste any better to me than the $5-a-bottle stuff. Call me uncultured if you like, but multiple studies have shown that wine experts can't tell the difference between expensive and inexpensive wine in blind tastings—or worse, that they find dramatic differences when the same wine is served in different bottles. Most incredibly, a 2001 study by Frederic Brochet had 57 experts taste white wine dyed red, and not a single one could tell that it wasn't red wine.
The Slate article points out not only that drinking cheap wine used to be the norm in the U.S., but that it still is in much of Europe. I can't say much about wine prices in Europe, but I still sometimes miss living in Chile, where we bought $5 bottles of wine when we wanted to be extravagant and $2 bottles for everyday drinking (and you can find even cheaper wine than that). Some of the same ones are available in the U.S., but at three or four times the price you'd pay in Chile; I can't quite bring myself to do it, even for nostalgia's sake. Now my go-to Chilean wines are the sauvignon blanc and cabernet sauvignon from the Trader Joe's Viñas Chilenas series, for $4 a bottle. The store has a bunch of other wines for $3-$7; a lot of liquor stores also have good options. Just avoid the grocery store if possible, since markups tend to be high there. (And try boxed wine. Really!)
That's not to say that none of the more expensive wines out there are worth the money, or that there aren't some really terrible cheap wines. Some commenters on the Slate article are outraged at the implication that there's no difference between cheap and expensive wine, but I don't think that's really the point of the article. It's more that $15-$20 has become the commonly accepted price for "everyday" wine, while there's a perception that $4 wine will always be undrinkable—which is sometimes true, but not always.
Palmer says at one point:
If hints of cassis, subtle earthiness, and jammy notes don’t interest you, you are not a lesser person. Wine is not art. There’s no reason to believe that aligning your tastes with those of a self-appointed elite will enrich your life, or make you more insightful or sensitive.
I completely agree. If you want to be a wine snob, that's fine too—go for it. But there's no reason wine has to be snobby.