I think the main problem with related post plugins is that they don’t seem to evolve as often as they could. I did a Google search and landed on Mr. Cutts’ blog where he explored some popular recommendations.
This is where it gets tough to find one that can compete with Yet Another Related Posts Plugin (YARPP). Competition is always good in the theme and plugin space since it forces developers to either innovate new features or add their own twist on existing ones. I think the related posts niche can easily be as popular as the SEO plugins and contact forms, since every blog could really make use of one.
Personally, if there was a complete solution that I could install on every blog I would be happy to pay for a premium solution.