Had a strange feedback, just a . On checking as I thought they had just made a mistake but this buyer usually leaves just a dot, there were 100 or more of them, with a few comments sometimes thrown in. Suppose I was lucky it was still a positive. Then I figured they must think a dot is good, maybe I was just so good there were no words good enough to describe the experience with me... so I did they same for them. After all it does look so good on my feedback. Needless to say I think after writing this maybe I had better put them on that magic naughty list we all have. lol
As others have mentioned, most people who leave this feedback are doing so because they (generally) feel like they have to leave feedeback, but don't have anything in particular to say, and don't like feeling obliged to write something (this is my personal interpretation, based on what I've observed, anyway).
It can come across a bit like a tight-lipped "no comment", though, so I also understand why it can be a less than desirable addition to a seller's feedback profile - the first one I ever got put me off-side a little, because it seemed strange to me to go to the effort of leaving feedback, but just leaving it at that... (this is in the context of knowing A or - for super special sellers - A+ takes just about the same amount of effort and actually conveys something, and for me, all I have to do is type one letter and use auto-fill to do the rest, while leaving FB in bulk - as in, I can hit one key, click one button and leave a several-word FB comment for 50 people at once, so I don't quite get the resentment and / or aversion to giving a little context to FB).
But, at least it isn't like some of the comments I have seen over the years, where people are actually angry at eBay / the seller for them feeling obliged to leave FB and make nasty comments to that effect, rather than keep their mouth shut with " . " 
(Interesting note, on another site I sell, people aren't limited to 80 characters, but they can also just leave a rating without a comment, an option a lot of people seem to like. The odd few will write a full-on review, several sentences long etc, but the majority are tired of writing the same stuff over and over again).
