What I really enjoy with this series is how I still have no idea what kind of ending we'll get. I think that's what pisses a lot of people off with it too. They aren't guaranteed their "Safe" or "Happy" ending, so they agonize over the possibilities and begin to hate the characters that switch sides or motives and force said "Happy" ending further from their grasps. The "train wreck" bandwagon I call these folks (it seems to be a big buzzword as of late).
I suppose there's some kind of consolation to not knowing the ending. Frankly I'd be upset if Lelouch didn't "win" in the end, he's such a tragic boy and never seems to get a break.
I could be wrong about this musing though. Honestly I would like to hear a well thought out opinion on why this show has become a "train wreck", cause most of the rants I read at places are just. "[insert character] is gay" "[insert character] needs to die" "I hate [insert scene]" "[insert character]'s costume is pure faggorty" and so on… Someone please enlighten me
Crossword got the jist of it, but to elaborate, the show really has an "identity crisis". On the one hand, it tries to act like a serious political drama, on the other hand it's a silly light-hearted school slice-of-life show. On the left foot it has a metaphysical undertone, and on the right foot a moral overtone.
Perhaps the biggest reason Geass fails as a series despite the entertainment value, is it tries to integrate all of these things into one whole; the school life plots could have been kept wholly separate from the political drama, but yet Lelouch's personal life gets dragged into his public one. The Geass metaphysics could have been a C.C.-only plot story unconnected to the Emperor and his political tyranny, and the Geass morality could have been confined only to Lelouch. Trying to weave everything only had disastrous consequences, with the metaphysics and school life ending up dropped and the political drama/morality getting turned on their head just to keep the train going.
The second big reason the show is a trainwreck is the characters don't make any sense. The only character who was easy to follow who went through a ton of development was Lelouch, except in the recent arc where I don't get what he's doing. Other characters like Jeremiah and Lloyd/Cecile simply stayed out of the moral department by being "mature" enough to make decisions on their own. Everyone else from Suzaku to the Black Knights to Schneizel didn't have enough elaboration on their personality for us to understand their seemingly confusing and arbitrary decisions. There's not enough internal monologue because the series is rushed, but personally I think the monologues were skipped BECAUSE the characters don't make sense.
Third reason is the fights have become more and more ridiculous. In early R1 the fights had the semblance of science fiction and realism, now it's almost fantasy in the way that individual pilots can command one-unit armies. From R2 on, all the robot fights in Geass resemble the kind seen in super robot shows like Gurren-Lagann or (I'm going there!) Gundam SEED Destiny.