Here is an (incomplete) rundown of the various 'similar/related node' type modules:

It should be pointed out that while related links is one of the oldest of the bunch, and has most features, it is not quite as flexible as people are looking for (which I think is the main reason for the other modules), and the UI is not always easy to understand.

Here is what I would like to see - a module that (on node creation/edition and also regularly refreshed on cron) provides a hook to allow various modules to add in some numeric factors for how that node relates to other nodes. These modules could have their own configuration (e.g. to only pull from certain vocabularies), and the central API would allow you to weight one module/factor against another (e.g. taxonomy similarity v.s. word similarity v.s. destination node votes or freshness).

It would basically then just dump this information into a table and provide it to views as a sort option (that would need to load a node from an NID passed in an argument to provide a 'relevant' sort). This would allow you to display it per content type or whatever. For bonus points there would be a way for Content Recommendation Engine or other modules with user specific input to add their influence to the final list.