One of the odd things about Resident Evil: Umbrella Chronicles was the omission of Resident Evil 2. As the apex of the series pre-RE4, it has a genuine claim to the title of the best survival horror game of all time (with its only real competitor being Silent Hill 2; survival horror series tend to go straight downhill after the second installment), so not including it as a main scenario was pretty baffling. Granted, it wasn't totally absent, with Hunk and Ada both getting their own bonus scenarios, but Claire and Leon were nowhere in sight.
Darkside Chronicles puts them back in the spotlight, giving the Chronicles rail-shooter treatment to RE2. Of course, because most blessings are mixed, it also hits up perennial series whipping-boy Resident Evil: Code Veronica.
That's right. Steve Burnside is back. This is not a drill.
It's awesome how they completely reused the character models from RE4, isn't it? Leon's even wearing the same combat belt.
In singleplayer Darkside, you can actually see the other playable character, and that character is actively contributing to gameplay; while they often pop up right in your field of fire, they're invincible and they're often holding zombies back, so it's a wash. One of my big complaints about RE:UC was that your "helper" character in the singleplayer game wasn't actually, you know, helping, so the active contribution is nice.
Another change between games is the addition of a sort of "shaky-cam" effect. As your character progresses, the camera whips around haphazardly just as your character's gaze shifts, resulting in stilted camera angles, blurring images, and blowing a lot of ammo on wild shots.
After playing it, it's odd how they've chosen to approach the new game. RE:UC is a full-on rail shooter, and makes no more attempt to frighten the player than a House of the Dead game would. RE:DC appears to be trying for more of a horror approach; there are more moments of attempted shock, and it plays more games with perspective, the way a particularly experimental FPS does. If you have any propensity for first-person-caused motion sickness at all, RE:DC's gonna smack you like a hammer.
Like RE:UC, the scenarios in DC are a sped-up retcon of scenes and sequences from the RE series. At E3, two scenarios were playable. The first followed Claire and Leon, moving from their first meeting in the streets of Raccoon City and following them as they frantically ran towards the RPD district house; the second is a revision of the first hour or so of Code Veronica, as Claire and Steve fight from the prison block to the military training facility on Rockfort Island.
I'm not surprised to see RE2 getting the Chronicles treatment, but I am surprised to see Code Veronica come back. CV is the last survival-horror game in the series's original chronology; after it, the RE series goes recursive, content to revisit Raccoon City or retcon the original game, right up until the release of RE4. It also stars Steve, who has a worse rep than he deserves, but who's nonetheless one of the most annoying characters in the series. I honestly cannot believe they brought him back, but it may involve tying up yet another lingering plot thread or two.
As with UC, Darkside Chronicles is confirmed to have a few unlockable scenarios that focus on the "dark side" of the series. Presumably, this means we'll see more levels devoted to finding out what various antagonists were doing during the series's huge timeline gaps. One of the posters Capcom's released has a shadowy figure in the background who looks a lot like Jack Krauser, and Ada's presence may lead to more revelations about the agency she's working for in RE4.
