twitter posts are limited to very few characters, so that already prohibits serious discussion. It's also transient in nature, not meant for drawn out discussions that could last for days, weeks, months or years. It's meant for announcements and "here's something I bought today" type of messages. Parsing twitter discussion threads is also difficult.
reddit threads are transient as well. A thread typically has a lifespan of 24 hours or less, maybe a few days on a slower sub, and in very active threads only the users who get in on the ground floor are likely to get their comment seen. Unless they "hijack the top comment for visibility," as they often have to do. If they make the mistake of saying something the collective disagrees with, their comment is collapsed and banished to the bottom. Even the visibility of the thread itself is dependent on people's votes. And like twitter threads, it's hard to parse reddit threads because of all the nesting, collapsing and moving comments (moving in the sense that when they are voted up and down their position on the page also shifts). You don't even get notifications for anything except direct replies to what you wrote.
reddit also by its nature encourages constant reposting and rehashing, and this in conjunction with voting conditions people to automatically accept and adopt popular views. Repetition + social proof = it must be correct.
On a related note, some have speculated that phoneposting has changed the way people post on 4chan. Interacting with a thread on a phone is much more cumbersome and encourages more shallow interaction, and posting on the go doesn't make people dedicate their time and attention to the thread as much as they would on a computer. Ban evasion is also way easier. This all leads to decreased post quality. Fast 4chan threads also encourage fast posting, which is bad for quality.