I fear the problem with flitering by thanks (or 'no thanks' if we used it) would be that people will thank or not thank opinions they agree/disagree with. Threads could thereby become very one-sided.
The idea of 'summary' posts which are marked and findable is brilliant except for the question of whose job that is, and when they're made. Would it fall to the OP? The mod? The first person to think the thread is getting a bit long? (and could the summary also be subject to biased interpretation?)