It's the same basic principle as newspaper articles, or the essay format you were taught in school: summary first, details later. Or make your points up front, then prove them.
That only solves a part of the problem, though. Highly upvoted long comments on Reddit are particularly frustrating because so many of them have content that's weak or just plain wrong. I'm convinced that only a small fraction of voters actually read them.
There is also a strong bias toward upvoting more visible content. Higher-voted comments appear first on the page, getting more exposure. For that reason I’ve made it a habit to read comments in bottom-to-top order, in order to artificially make my votes more fair.
That only solves a part of the problem, though. Highly upvoted long comments on Reddit are particularly frustrating because so many of them have content that's weak or just plain wrong. I'm convinced that only a small fraction of voters actually read them.