I don't really care about whether it showed anything to get excited about the main game--which it did, to me, I might add--I simply found it fun.
There's so much media about the game, if someone wants to find out more, then they have a chance. There's literally an anime, a movie, tons of trailers, videos, everything.
You're correct that if someone really wants to know more, theres tons of media for the game. The problem is that there are a lot of people who will either watch or play this demo once, decide its bad and move on. That's it. They won't give the game a chance beyond that and as someone incredibly hyped for this game, I find that frustrating.
Even with Duscae, as early as it was, there were still moments where all the elements came together just right. Fighting a group of enemies with party members in this huge world with beautiful animations, amazing music, and the sun setting in the background. Walking into the Chocobo ranch and having your party members separate off to do their own thing, Discovering a dungeon and fighting a bunch of goblins to get a really cool summon at the end. The new trailers today do this as well and ,imo, show perfectly what the game will be like. These insane fights with good graphics. Traveling with your party in cars, boats, chocobos, etc. Stopping at a rest point, playing mini games, etc.
I know Tabata has said that what we were getting today was a tech demo but if they were gonna do that, I think they should have just provided something like the TGS 2014 demo. At least in that, you have your party, can see day night cycles, weather, and you get to test the battle system.
I really just want other people to be excited and enjoy it as well because I feel this game might be something really special. Maybe I'm being dramatic (there was a huge event and all) but impressions are extremely important and it sucks when people miss out on anything great because the first one was so shitty.