I don't really like considering the two seasons of Code Geass as two separate series, but for the purpose of this site, I suppose I have to. R2 is, of course, a direct continuation of the story from Code Geass: Lelouch of the Rebellion, taking place after only a short time leap and neatly wrapping up the cliffhanger ending on which the first season left off (and early on, too--no waiting for payoff). That said, while this season could feasibly be picked up on by itself (episode 1 of this season was actually the first I ever saw, which confused me greatly for a while), the big picture isn't going to make a whole lot of sense of anyone who hasn't seen the first season.
Now, since I employed an alternate review structure in my season 1 review, let's see if I can make my traditional format work for this one.
Writing... hasn't faltered at all since season 1. In fact, towards the very end of the series, the story is stronger and more original than ever before. As stated in my previous review, I believe that Code Geass has what is likely the single most powerful ending of all time--for me, at the very least; I can say no wrong about it. The battles are even more explosive, entries more dynamic, and interactions more dramatic than ever before--something which, if you saw season 1, shouldn't seem possible. Characters all act with their own drives and motivations, each wearing his or her own mask in trying to change the world around them, and the end result is a bittersweet (but enthralling to watch) insight on human relations.
In all, I like to think of R2 as an "amplified" version of the first season--the politics are more intricate and engaging, and the drama more... well, dramatic. However, the school scenes did not escape this amplification--they're wackier and more invasive than ever. Which, again, doesn't bother me--I have no problem having a couple school comedy scenes intermingling with my epic political drama. If you are one of those people who found the school segments hard to stomach in season 1, though, know that you'll only find more of them in season 2. Though I'd like to think that if you made it all the way through the first season, you'll have a hard time not continuing the series. That's my hope, anyway.
Art hasn't changed--it's just as angular, shiny, and bishonen as ever. If you like shiny, pretty things then you'll enjoy some very fluid animation and an interesting and unique art style. If not... well, the show's going to be jarring for you, to say the least. It's essentially the battle between old Slayers and new Slayers.
Sound also doesn't really vary between the two seasons. All the original voice actors make reprisals, and new characters are all performed appropriately. As far as English goes, I'd say the poorly-cast voice roles are exclusive to season 1 characters--that is, none of the new characters introduced in R2 are poorly voiced (though some of those from season 1 are still around). I cannot myself speak for the Japanese dub. The soundtrack is the same one used in the first season, with a couple of new insert songs sprinkled in here and there, and it still works.
Code Geass is still my personal favorite series of all time, and between the two, I actually think season 2 is the stronger one, because... that ending. That goddamn ending. Beautiful. Anyway, despite the series matching all of my loves and interests flawlessly, it's not something for everyone, as the fiercely split opinions on the show will no doubt tell you. Watch the first episode or two of either season and you'll know what to expect from the whole show--many series take a little while to hit their stride and give you a good feel for the overall show, but Code Geass is not one of those series. It hits the ground sprinting and doesn't lose momentum--an incredibly enjoyable ride for yours truly, and I hope one for you, as well.