It seems as though @Parbury and @cardinal tried to address this problem over a year ago, in April 2011. It was not resolved, and @Cardinal politely tried to broach the matter again, more recently. However, that question was closed as a duplicate, and referred back here.
I don't believe that conversion to Community Wiki is a good idea. The answer given by @mbq (and the answer comments that followed) address that adequately already. Situations like this Comments that are actually answers need to be resolved though. Otherwise, there will be more of this Comments that are actually answers which benefits no one.
Whenever one notices a good answer to a question in the question comments, and then notices it again the next day/week/year (no, that isn't well quantified, I realize), one should feel free, though not obliged, to use that comment as the basis for an answer. It would be appropriate to mention the user name of the person who made the comment initially. The answer should be written up decently, not just a copy and paste or a link. Expanding on it a little would be nice, but not mandatory. Community policy should then be to up vote said answer if it is worthy. Voting should not be biased by the fact that the original idea was provided by someone else. There should be no shame brought to bear on the person wrote up the answer.
Moderators don't have time to fix everything. And points are often earned in far less productive ways! While there are not a large number of situations as the one described by @naught101 in my second paragraph, they will accrue over time. This course of action will prevent future build-up. Perhaps it will have the secondary effect of encouraging users to post answers rather than question comments. The goal is to encourage the users who have a good answer in mind, but are hesitant due to shyness, or awareness of the time required to write up a decent quality answer. I feel both, quite often.
- No, I am not being quarrelsome! I strongly agree with this: Comments that are actually answers as the reason there is such an abundance of comments that fully answer questions. I mostly attribute that behavior to the fact that users of this website, and many other StackExchange sites, have uncommonly high levels of moral and intellectual integrity. Don't any of you deny it! You know it is true. Well, "moral integrity" is an overstatement. Better: A sense of fairness, or merit-based preference. Possibly a tendency toward modesty, maybe antipathy for the seeming-appearance of "glory hogging" behavior. I have some basis for my conjecture. It is empirical, personal observation, I admit. Given the difficulty in quantifying human behavior, I am willing to go out on a limb. There are Q&A sites where network homophily and contagion (not that there is anything wrong with that, for a more "social" site...) is much more in evidence than SE. On such sites, I have observed that comments on questions are few and far between. Incidence of comments that answer the question are close to nil. This is true, despite the fact that comments on questions are allowed. They aren't implicitly discouraged by site policy, either. (It becomes problematic when questions are not closed, as dozens of answers pile up, over time.)