What I think we're looking at is a tendency toward a desire for higher originality. Take a look at rebus puzzles sorted by age, then scroll to the bottom of the page. As you scroll up, watch the scores, and you'll see what I mean. Initially, they scored highly, then slid down quickly.
Let me ask this question: how many triangles are in this pentagram shape?

If this were the first puzzle of its kind that you'd ever seen, then I imagine you'd probably upvote it: It's interesting the first time. The second time? How many triangles are in this figure?

By now, you're bored. Even if it's the first time you've ever seen a puzzle like this, you're still probably thinking "another one? Really?" Thinking back on the first puzzle, the votes are now inconsistent. Why upvote one and not the other?
People begin holding puzzles to higher standards of originality. We seem to have a bias toward new types of puzzles, even when these new shiny puzzles aren't all that high quality in the context of others.
In the long run, it's probably alright to let this bias run its course. For each type of puzzle we see, we'll come to expect originality and high quality. I think our standards for these puzzles will converge and be consistent long-term.
As a case in point, I'm going to take rebus puzzles as an example, as linked above. Many of the first rebus puzzles posted were upvoted a ton. Theeeeen after about seven days, we hit a stack of negatively-scored posts. Why? If I had to guess, I'd say people got sick of dumping rebus puzzles. When it comes down to it, a lot of them are really mostly the same.
I'm confident, though, that if someone were to post an intricate rebus puzzle, or one which contained some unusual or nonstandard variation, it would receive upvotes.
Do we need to do anything about this bias? I'm not sure. Let's split it up by new users and regular users:
- For regulars, I'm not sure it's a serious issue. Regular users know when they're being original and when they're not. We know when a puzzle we're posting has new value, an unusual twist, or odd surprise - and when it's just a pulp paperback puzzle. If you'd see it, and can imagine yourself going "another one? Seriously?" then it's probably not original.
For new users, it gets sort of tricky, and I'm not totally sure what to do about it. We definitely want new users to come and stick around a while, and that usually means having a constructive experience on the site. It can be hard to get "constructive" and "downvotes" to mix well.
Here's what I'd recommend: if you see a lackluster puzzle, and it's coming from a new user, leave a friendly comment. Give 'em a welcome, and let them know that we see a lot of these types of puzzles, but if they have other new or interesting content they'd like to post that they should feel free to.
I'm not sure this covers everything, but it's a place to start, and I think this is the pattern we're seeing.