I have seen many questions and answers on Programmers.SE and on Stack Overflow that I would consider older (maybe 2 years and more) and that are not so good compared to more recent posts. The funny thing is, they have a lot of votes. Today, I see better questions with a lot less votes. Is this a matter of people up-voting less these days?
Quality of questions and standards is surely a factor, but consider that a question that's been hanging around for two years has had a long time to accumulate votes. As time goes on, people will do searches that turn up older questions and still vote them up if they're good. And when someone asks a new question that seems familiar, somebody is likely to dig up the old one and vote to close the new one as a duplicate, thus directing more people to it.
If you want to judge the quality of a question, you have to consider both the number of up votes and the number of views. A +30 question that's only had 100 views is more likely to be of high quality than a +30 question with 3,000 views.
Yes, that's true.
Programmers had a troubled start, fully documented in this answer. In the beginning the site was called "Not Programming Related" and it was a very relaxed place where questions that didn't belong on Stack Overflow found a home. Inane (but populist) questions were migrated here from Stack Overflow en masse, carrying ridiculous amounts of upvotes with them, and for a short while NPR was the one place in the Stack Exchange universe where you'd go for mindless fun.
However, it soon became evident that people flocked around the bike shed ones and ignored any serious questions or answers, and the site was quickly filling up with crap. Which led to a somewhat sudden and drastic scope change, the site was essentially rebooted to be what it is today. The redefined scope meant higher quality standards, which for Stack Exchange usually translates to less down votes and more up votes. Combined with the fact that down votes on questions became free about a year ago, people are down voting a lot more liberally than in the past.
Of course you should also consider that older questions had obviously more time to gather up votes, they aren't all bad, newer questions will get there in time. But the main reason for the few whales that remain was the undisciplined culture of the early days, people would upvote just about anything. To give you an idea, one of that era's questions was "What's your most controversial programming opinion?", which had 391 upvotes and 414 answers.
As time goes by we become even more sharply focused, and we set our standards even higher. Useful and well research questions may not get a lot of up votes, but they do get a lot more visibility and quite more useful answers than a couple of years ago, as they are a lot easier to discover when you don't have to look for hours amongst an ocean of pointless questions like "What's your most controversial programming opinion?".
@Rachel probably has some statistics to back this up, but these days, highly voted questions are usually either closed or converted to 'Community Wiki'.
That's why it's harder to accrue reputation points now, than it was at the beginning.