I think it's a good idea to make reputation on a large Stack Exchange sites more like real-world currency. At least in a sense that it should have it's own inflation rate.
For example, answer or upvote year ago should cost less in an absolute value than tonight.
The reasoning is simple. Community's knowledge base gets bigger and bigger every second, and it's harder and harder to ask and answer a good question that haven't already been there. I think, given the same tag, gaining N of rep. say 3 years ago was much easier, because there were tons of simple unanswered questions like about date conversions on SO.
Maybe it'll be a good idea to slowly increase rep. cost of all actions as tag (question's tags in average) gets older and bigger? For a 10% in a year or so.