I haven't really read any guides on Japanese grammar (except occasionally looking up pages of Tae Kim's when I need something explained), so I became quite confused when I saw things on Twitter like これすごい (instead of something like これはすごい). I also noticed people seem to do the same thing with verbs, like saying アニメ見た instead of アニメを見た (that's probably a bad example, but it's the first thing I could come up with).
So what is up with this? Is there some sort of pattern behind it I can follow? At first I thought it was something done in a casual setting (like a Tweet), but I feel like I've seen the same thing done in more polite Japanese.