On the surface, this looked great - promising setting, themes, characters, world. Unfortunately, all things that were clever about Season 1 were scrapped in favor of... well, I don't know, really. Shocking the audience? Misplaced fanservice?
The most glaring and obvious problem is that the cool antihero protagonist that was so refreshing in the first season got derailed into a wangsty Canon Sue. No clever tricks, determination, character growth or development, nothing but the endless loop of Oh Noes Nunnally over and over. And yet for some reason the camera refuses to leave his immediate proximity, so while the other characters and settings might have interesting potential, all that is ignored in favor of more Lelouch-angst. And yet despite the fact that the only thing consistent about the character is his self-centered wangst and hypocrisy, every major character magically finds him loveable and sympathetic even when he's unrepentantly destroyed their lives. It's like reading through some particularly bad piece of fanfiction.
What the narration then ignores are things like character development and narrative causality. The clever tactical ploys of the first season were pretty much nonexistent, relying instead on beam-spamming Super Mecha duels (y'know, the very thing that people complain about with Gundam?) and liberal application of plot-induced stupidity to the opposition, or just reiterating the exact same plans used in Season 1. Deus Ex Machina doesn't even begin to cover this. The Geass mystery that seemed so important was given no resolution and pretty much written off as a footnote.
Characterization was about as consistent as Swiss cheese, with characters seemingly completely forgetting what they'd said or done just one episode ago and doing 180 turns with little to no exposition. Surely they could have spent some of that time wasted on fanservicy hijinks or the Endless Angst Loop fleshing out the supporting cast? Forget actual development - the more screentime a character got the less consistent their actions were, and the only characters who didn't act like they had multiple personalities were the ones too insignificant to matter. If you're going to have characters acting in ways utterly opposed to everything we've seen about them up to this point, you need to spend some time explaining to us why. Part of this can be blamed on the introduction of a whole batch of new characters while refusing to leave any of the old ones, resulting in neither old nor new characters getting enough screentime. Old characters who had outlived their story role hung out in the background but were not given any new purpose, while new characters that looked promising in concept never got enough exposition to actually live up to that promise.
The only really positive things I can say about R2 is that it's got pretty animation (mecha combat falls short of Macross Frontier, but it's not like Code Geass focuses much on that) and the background music, particularly the vocal pieces by Hitomi, is gorgeous. It generally manages to surprise the audience in its unpredictability, but unlike Season 1, often crosses the line into unbelievability instead. "That was unpredicted but it makes sense" is different from "that was unpredicted and killed narrative causality".
Overall, the ideas, characters, and setting were interesting, but the script appears to have been written by a particularly indecisive committee of retarded monkeys. It's not a fault of the premise itself - the spinoffs such as games and novels manage to take those same settings and characters and actually make them interesting, or at least make sense, so I'm not sure why the supposedly high-budget anime fails so badly at doing so. It is not, of course, universally bad - there are moments of cleverness and good writing, but those moments are overwhelmingly outnumbered by the bad writing.
R2 is fine to watch for the lolz if you turn off your brain and try not to think too hard about it, but those expecting the cleverness of Season 1 should probably stay away. If you're really into the series, read the novels/manga or write your own fanfiction, I guarantee you it'd make more sense than this drivel.