I tried to put down some thoughts for people who actually havent played Xenoblade (and asked questions that are obvious for Xenoblade Players), so they maybe can understand what that trailer was all about. (I try to use no spoilers, but if you consider game-mechanics as spoilers I probably would dont read it.)
Combat:
Im really surprised, so many people disliking Xenoblades battle system ._.
I actually loved it, with the quick time chain combos, topple and all the other things you had to set up with your team mates and actual team-play to even be able to damage some enemies. Thats what bugged me the most about Xenoblade, not being able to do that teamplay with actual people/friends.
For the people who thinks its like Monster Hunter: Its not at all I tried Monster Hunter many times, and I just cant make me like it. The combat is just too clunky for me. Its closer to Phantasy Star Online, but still VERY far away from that. What I really like about the combat system, too is that you can always choose one attack in advance so its not about timing your button presses right (which is one of the reasons Monster Hunter feels so clunky for me) its more about choosing the next one as fast as possible (which was a pain with the Wii-mote. Classic Controller 4tw). (Actually a lot of Action Games use that mechanic, too, just hidden in the background if youre screaming MMO! right now again.) It felt very unique to me through the whole game, and when you think you got the hang of it, the game just throws another mechanic at your face, you need to use, to defeat the enemies. But I guess thats the problem, many people have with it. Its new. Its fresh. And they dont know how it works yet, and dont want to learn it. And that exactly is what I love about it. Even if you try to avoid mechanics its eventually gonna blow up in your face with the last bossfight in Xenoblade
Why not stay in the mech:
Speculating on Xenoblade Lore Id say the dolls/mechs will damage your character (maybe also story wise) in some way the longer you use it. So you might now want to push it to the limits.
MMO:
It will definitely not be an MMO. Online Co-Op though wouldve already felt very natural along the story in Xenoblade though. Maybe theyre just going to make you able to BYOG (Bring your own gear) and chose one of 4 (or more, like in Xenoblade) characters so every character is Story related. We always saw the same Brown haired Player (Main Character?) one so far, so Id say thats a possibility. Not sure how theyll manage the Abilities though. Probably still just stick the abilities to their Story-Related Characters (like in Xenoblade).
Background information: In Xenoblade you had three Team Members you could choose out of maybe 8 Characters? It was possible to switch them out, any time. You could choose which one you want to play, the other two would just be controlled by AI.
Its not even a given that its 4 Player Online with this trailer. It looks exactly like Xenoblade did, and Xenoblade was Single Player only.
Teamplay:
Is really important in Xenoblade. There are enemies you cant damage at all if you dont topple it. As far as I remember you have to do an attack (I dont remember the names) Orly, then your mate has to REYN TIME in the next ~5 seconds. Then you have to Yarly in the next ~4 seconds again, only than the enemy is vulnerable. The Attacks need to be done from certain sides of the enemy, so if yothe enemy is attacking you, you cant use it at all! As far as I remember these three Topple attacks all have Cool-downs so you might not want to throw them out early (befor your mate used REYN TIME or in a stupid situation, it would be too easy if they didnt have CD though. Mechanics like these take a little time and effort for some people to get the hang of it, since its actual Teamplay (with AI-mates :/ ) and not button mashing, AND they are exactly the reason why many people want Online Co-Op.
QTE:
Its not really QTEs this could be something like the Topple I just explained. Just easier with a huge symbol so you dont miss it (I dont think its that, though). It was also used for Chain attacks (which dealt a lot of dmg / healed up your party if you had a healer (No Healer-Rikki-Team 4tw HEROPOOOOON) while time stood still. You dont have to hit these, but they come in handy later in the game, and are actually fun to use, when you think about how to chain the attacks (Thats why the time stands still when you use chain attacks).
The scene:
The emies level was far under the Parties level. And theres probably more going on than youd think (havent looked at it too closely yet, but maybe he just plays good & with topples, too?)
Since the first enemies level is so low (Level 13, was it?). The Big one is probably one of the enemies you cant really encounter when you first visit that place
Thats all I could think of right now 
Last words:
Seriously guys, play Xenoblade Chronicles.
