Okay, after absorbing all the info from TGS about the playable demo, I have a pretty good idea of the gameplay systems in the game shown so far and it's time to feel the LOVE!
Before we begin, here are two rules:
1) No quality comparison arguments about other RPGs. We don't need "Blue Dragon looks better" or "OMG WHITE KNIGHT STORY!!!" or "Lost Odyssey and FFXIII look better than this!" here. If you want to say that you think another game is better, and feel that is all you have to contribute to this thread, hit back instead.
2) No Scamco nonsense here. I know I'm one of the most vocal against Namco Hometek on this forum, but once again, we don't need that crap here. However you feel about the name change or whatever potential changes - take it to another thread.
Okay with that out of the way let's begin. From the Eternal Sonata PR, here's what they said:
Key points:
- unique monster morphing system
- a hybrid of a turn-based and action battle system
- a special attack feature dependent on light and shadow
But what do they mean?! After looking at the MS Conference trailer and the 3 available gameplay footages available (Gamevideo, Xboxyde, IGN), this is what the battle engine is like:
Enemies are visible on the map, and you run into them to start encounters. The transition is seamless with no loading. Really smooth. Each battle field has really nice lighting with areas clearly laid out in shadow. This is simply a cleverly natural way to use the environment and graphical effects to divid the battlefields into light areas and shadow areas however...
The game is turn-based, with each character's turn having a bar on the left side of the screen that resembles an AP bar from strategy rpgs. As you move on the field, use items or use attacks (normal or special), AP is drains. You can pull off combos by attacking repeatedly as long as the bar isn't empty too.
You move with the analog pad, A pulls off a normal attack, B defends, X uses the selected item which you select with L and R shifts. Finally, Y pulls off the character's special attack. This is the first part where light and shadow comes into play. For example Allegreto's normal special attack is Sun Slash, but if he moves under the cover of shade, it changes to Void Edge. Hence where he moves determines which special attack he will pull off.
Here are examples of the special attacks changing:
Sun Slash to Void Edge (Allegreto)
---------> 
Orange Cure to Shade Comet (Polka)
---------> 
Blast Fire to Solid Shooter (Pete)
---------> 
When it comes to the time for an enemy move, the enemy too has a bar that drains as it moves and attacks. Since the computer does not need to stop and think, this plays out a lot faster. This is when the light and shadow comes into play again. Enemies have a light form which is smaller and cuter, and a shadow form which are much larger and more aggressive looking. Depending on whether their location on the field is under shade or not, their forms morph dynamically in realtime. It'll be good to lure an enemy from shade into the light to give yourself an easier time.
Here are examples of the transformations:
---------> 
---------> 
Out of the 3 characters in the TGS demo, Polka and Allegreto have melee attacks so you have to get close to an enemy to pull off offensive moves, but Pete uses a crossbow and is a range character. The "realtime" part of the PR refers to how all attacks are realtime relative of direction and location. So you can waste actions by attacking thin air if you're not careful about placement and the direction the character is facing.
Also attacking enemies from behind will deal more damage than attack from the front, and if you are able to pull off a long enough combo, there are also finishing strikes at the end of the combo string that does much more damage. Overall the game is shaping up to be a REALLY impressive RPG with a great battle system, GREAT environments and NO LOADING!
Those that have played Baten Kaitos should know what Tri-Crescendo is capable of, but it seems they've really outdid themselves this time. I really hope MS puts this demo on Marketplace, especially since the demo is tailor-made for TGS (the characters even introduce you to the game and Tri-Crescendo at the start) and it would make a great surprise to a rather disappointing Bring it Home campaign so far...
Update:
Now you can get HD (subscribers) or SD (free members) footage of Trusty Bell with audio and a good player at the controller here - http://www.gamespot.com/xbox360/rpg/trustybellchopinnoyume/media.html
Especially recommended is the second video for example of the realtime enemy morphing mentioned.
Update 2: The game also features a combo system!
The game's combo system is represented by a chain bar (displayed as a bunch of stars starting at the bottom right of the screen) that grows as you use only regular attacks. At first each 4-hit combo will increment the bar by one star, and later it grows only with each successful 8-hits, and so on.
The combos stay even after enemy turns, so as long as you don't use a special attack, the chain keeps building. The combo chain stays even if you use non-offensive specials (like Polka's Orange Cure) and items. It appears that the chain bar might be linked to the power of special attacks, because with the bar at 32-hits, Polka's Shade Comet did over 800 damage, but afterwards it only did about 120 damage.
This system is especially visible on the 4th Gamespot video in the battle against the boss.
Sources:
Xboxyde - http://www.xboxyde.com/news_3507_en.html
IGN - http://media.xbox360.ign.com/media/761/761569/vids_1.html
Gamevideos - http://download.gamevideos.com/6325/gv.com.trustytgs_320x180.flv
