For me it's someone who is super unpredictable when texting, like sometimes they'll reply right away and other times they'll read a message and not reply for a few days. Basically someone who's a bit flaky and you're always kind of afraid that they're going to ghost you at any time (I honestly think that's just me overthinking and being paranoid though). I can handle waiting hours for a text, but when it happens really often I just feel like we're wasting each other's time.
Of course I know that life doesn't revolve around texting and being on your phone, but it does really make me question whether they're truly interested if they don't seem engaged in talking to me, or it seems like it's a hassle for them. The opposite is also annoying though, where someone is super clingy and get annoyed if you don't reply within like 2 hours. I feel like you need to strike a balance between the two.