Geass isn't so much shit as it is dumb. Really, really, REALLY dumb. This makes it charming, but charming in kind of a really, REALLY guilty kind of way because of its highly exploitative nature.
It's basically the perfect storm of bad characterization and insipid, childish exposition about the nature of humanity (IS IT RIGHT TO BE STRONG DUR HUR HUR), punctuated by a couple moments of just awe-inspiring pandering/shock value (Nina and the table, the Euphinator).
It's hard to articulate what, generally, makes Geass so stupid. The way I prefer to describe it is that if Brendon from Home Movies (excellent show) was a weeaboo and decided to direct an anime, Geass would be the result.
So why does everyone watch it? Because even on its worst days, Geass is still better than pretty much every other anime on TV these days (except Golgo, because he's a fuckin' G). This is not to say that Geass is a great show -- it's simply that its freakshow factor is infinitely more entertaining than all the other moe/insipid kiddie crap currently airing.