The internet genres that go for clever but shallow wit and/or maximum emotional rev-up don't fit well here. The pleasures of surprise, recognition, and indignation that they evoke run at different frequencies than the intellectual curiosity that HN tries to nurture. The latter needs material which is more reflective and longer-lasting. As PG wrote somewhere, the thought processes are quieter and slower. Snap reactions thwart them.
It's not that those other genres suck—they're often delightfully clever. But their influence takes HN away from its core. Experience has shown that such high-frequency, low-amplitude stuff drowns everything else out if allowed to, so we don't have the luxury of allowing everything equally. For HN to thrive, we have to clear a space for more substantive material. That's hard and constantly being encroached on. But one fairly easy win is to rule out sites that play the other game. In our more fragile ecosystem, they count as invasive species.
theoatmeal.com and others have passionate fans who will disagree. But one ought to separate the fact that one likes something from where it belongs. I like ice cream, but not on meat and vegetables.