Perhaps others are better about this, but I, personally, have not been very consistent about upvoting good answers. This is not for lack of desire; rather in the heat of the moment (servers down, etc.), I'm not thinking about formally recognizing good answers, I'm simply looking for a solution as quickly as I can.
On the flip side, when I'm just browsing, I'm rarely trying to solve an issue, so I lack the immediate "I just tested this method and it worked for me" experiential basis upon which to upvote (or downvote) anything. Also, it's rather difficult to remember many of the questions/answers that I visited previously in the heat of the moment.
It would be really nice if I could "flag" answers for future review. The flagged answer would be put into a queue for me to visit later and decide whether to upvote or downvote.
I want to recognize good answers, but I also want to test the answer for myself before I upvote it. This would provide an easy way to revisit answers later after validation.