SO, before I (re)write this, I need to say how annoyed I am that I had it fully written-out, it was saving periodically, my browser crashed and it turns out that none of it saved. Whoops. So, this is a watered down version.
Two-word emails.... just don't do it. And for that matter, one- and three-word emails are terrible as well.
Imagine you just applied for a job via email, or had a promising first date (but since dates aren't really called dates anymore, we'll downgrade it to a 'hang out'). You painstakingly craft a professional email to send with your resume and cover letter. You call your friends to make sure "this doesn't sound too crazy" when you tell him/her you had a nice time.
In response you get: "Thanks" or "Ok, thank you."
How is that supposed to leave me feeling? I would rather you not reply at all. It is so up in the air. How about something like "Thank you, I have received your application and it will be reviewed shortly. Thank you for your interest in our company." That took me all of about 15 seconds to type. Or maybe, "Yeah, I had a great time, too. That restaurant was really great." Also took me about 15 seconds to type.
Unless you intentionally want to convey negative feelings or disinterest, then the "ok," "k," and "cool" all have to become a thing of the past. In the case of emails/messages between close family members, Microsoft Outlook correspondences, or other instant messaging services, brief can be totally fine.
I just know that whenever I get a really short response to an email, I get the impression they don't want to be communicating with me. Sometimes people are too busy to send something substantial, but that doesn't make the crappy one-worders ok. I would rather wait longer for a more substantial response or be completely blown off with no response, than get a short reply.