Weird plot shit is a derivation of weird puzzle shit, as there are no actual puzzles to figure out in order to solve the game - It's the nature of the game itself and its plot that makes it so confusing. It is almost never referenced in its single appearance, Homestuck, but it is very clearly and obviously there. None of the characters feel that the situation they are in is weird plot shit, which is probably why it has never been addressed as such. In fact, many of their problems are more precisely rather weak, more direct forms of weird puzzle shit rather than weird plot shit. Some characters are not aware of the consequences of the weird plot shit, others are too focused on their own problems to address it, one is in utter disbelief of it, and the last treats it all as normal and thus has no reason to question it. To the MSPA Reader, however, a lot of weird plot shit is about trying and making sense of it. If there is any sense to be made out of all the weird plot shit, it just may prove to be critical to beating the game.
The color sequence of 4redobjects, followed by a blue and another red object close together, followed by a green object and finally a white object (as seen in Seer: Descend with the curtains, here, and Cascade with the chapter buttons).
Basically everything else that has to do with trolls and cherubs.
The mysterious lab underneath Rose/Roxy's basement.