I think there are a lot of reasons that RPGs can stumble over the 20 minutes of fun in four hours thing. Many of them go beyond the game mechanics. For instance, I think LostSoul's comments echo many of the worst sessions I've played, particularly at conventions. The party wants to do fun stuff, but the DM/scenario writer is too obsessed with making you walk from one part of the plot to the next, picking up random clues along the way, until the big reveal at the end. Obviously, that's a problem with adventure design, not the rules. I've played that "game" in D&D, Shadowrun, Spycraft, etc.
As a lot of people have said, the big issue lies in DM training. A lot the advice out there falls into a few pit traps:
1. It's way too simple. "Dangerous monsters make for exciting encounters!" Really, you don't say? This is bad because maybe it helps beginners, but it primarly teaches people to ignore advice in gaming books.
2. It sets the bar way too high. The Campaign Sourcebook and Catacomb Guide from 2e fell victim to this. It basically said that combat is bad, avoid it all costs, and players who aren't in character 100% of the time and who are the equivalent of trained, professional actors, are a waste of space. Um, yeah. How many groups actually act that way? Maybe 5%? Yet this book depicted it as the norm.
3. It sounds nice in principle ("Everyone should wear hats when they're in character, and take them off when they're out of character!") but in practice it goes over like a lead balloon ("I am not wearing that @#^$@#$ hat again you %$@#$#@ !@$@#! son of a @$!@#$#.")
Realistically, I think the vast majority of gamers like "wasting" time at the gaming table talking about movies and videogames, or making fun of the designated spaz in the group. The problem is when the game starts and nobody gets to have much fun, or if socializing replaces the game because the game is boring.
I don't think D&D (or any well designed RPG) is inherently doomed because of the 20 in 4 issue, but I do think it points to a larger issue that the hobby has faced since its inception.