You definitely won't get away with posting something like "Great!".  On most other forums that would be fine, and it seems to have been fine in the early days of bitcointalk, but I think forum standards changed once most people started getting paid to post here.  Since some campaigns don't have character minimums, if mods didn't delete such short posts as the one you mentioned, everyone in a sig campaign that didn't count characters would be posting "Nice!" or "+1" a hundred times per day.  And just think about how much more unreadable this forum would become if that were to happen.

So I'm pretty sure that's the rationale behind why one-liners and posts that are insignificant get deleted--and it's in the rules, too.  Just make thoughtful posts (like the post you made here, OP) and you shouldn't have anything to worry about.  If you want to "like" a post, give it a merit if you have any.