This particular point is demonstrably false, at least as a general one: people acquire taste for foods and activities they previously disliked all the time.

I've done this a lot. Each time I did, it wasn't because I forced myself, it was because I saw some new attractive thing in those foods or activities that I didn't see before. Perception and enjoyment aren't constant. People are more likely to try new activities when they are in a good mood (for instance). Mood alters perception. In that sense I actually agree with Villiam_Bur. You can get more people to become 'rationalists' through engaging and fun activities. But you have to ask yourself what the ultimate goal is and if it can succeed for making people more rational.

However, an essential part of being instrumentally rational is figuring out the right amount of computing power to dedicate to a particular problem before acting.

The most powerful 'subsystem' in the brain is the subconscious system 1 part. This is the part that can bring the most computational power to bear on a problem. Making an effort to focus your system 2 cognition on solving a problem (rather than simply doing what comes instinctively) can backfire. But it gets worse. There's no 'system monitor' for the brain. And even if there was, if you go even more meta, optimizing resource allocation for solving problem X may itself be a much harder problem than solving X using the first method that comes to mind.

Except Reddit is clearly winning, in the "rationalists must win" sense, and Slashdot has all but disappeared, or at least has been severely marginalized compared to its late 90s heydays .

I know it's an extremely subjective opinion, but it seems to me that the slashdot system reduces spread of misinformation and reduces downvote fights (and overall flamewars). As for why slashdot has shrunk as a community, I suppose it's partly because reddit has grown, and reddit seems to have grown because of the 'digg exodus' (largely self-inflicted by digg) and the subreddit idea. Remember that there used to be many news aggregators (like digg) that have all but disappeared.

The idea here shouldn't be "let's adopt the most popular forum system", it should be "let's adopt the forum system that is most conducive to the goals of the community." And we have at least one important data point (Eliezer) indicating the contrary.