I am very much happy with it. And while I'm not too interested in Leblanc as a character, she is a much needed mage for the team being put in front of the player. However, with the additions of Irelia and Katarina to the story, who I assume will be your main party at the end, there is one downside I can see, being a lack of a character with any real healing magic. Because of this, I feel like Soraka or Sona would have been a welcome addition to the game as one of the four mains. However, I won't make that judgement yet since Irelia only had two abilities in the demo and both Leblanc and Katarina don't even join the party. Sure, Cnal Bel is in your party at one point, but he had very little magic anyways.
Next, mechanics. I feel like the spell selections of the various characters are good. Riven now having a cooldown system makes a lot more sense for her and I assume Katarina will be using a similar system. Battles do get a little too hard in this demo, as I was complaining about Irelia being extremely weak when I got her, getting destroyed by virtually every monster I found. Until I got Dyuu, battles in that cave were disasterous. And even after I got him, they were still really hard. I feel like the Paralysis status effect is getting outright abused at that point and in battles where all your opponents are tentacle monsters, full party paralysis is a very real danger.
If I may make some suggestions, Paralysis needs to be nerfed big time. Right now, paralysis lasts a very long time and seems to half your defense, making you take far more damage. When paralysis is already lasting 4-6 turns on average, the defense loss isn't necessary. Full party paralysis might as well be a Game Over. Right now, Paralysis seems too powerful considering you cannot get any accessories currently that block it. Also, the only accessory you get for free is a Poison-preventing charm. As far as I've seen, none of the monsters currently in the demo are poisonous. Could we please get that replaced with something more useful like, say, Blindness or Paralysis?
Also, the boss of the mines before the end of the demo was way too powerful. My party levels were Riven 10, Irelia 9, and Dyuu 7. And that boss took way too little damage, had way too much health, and hit way too hard. Keep in mind that, at this point, my party's equipment was Riven with the Flowing Waters and Relentless enchantments, Linen cloth armor, and the Necklace of Poison. Irelia had just her basic stuff; the Guardsman Sword, Strength+ enchantment, and the Enchanted Cloak. And I nearly got destroyed. I didn't buy any enchantments or the like, since nothing gives enough money to afford them at 5000 gold for the nice ones, but I do feel like better armor needs to be available and soon. I felt incredibly weak in that dungeon, so you guys may need to rethink the items scattered around, perhaps add some chests or the like.
As for secret rooms, I only found one; the one with the Flowing Waters enchantment, so if there's more, they're a little too secret. So if there's supplies I could have gotten in those caves, I didn't find it, which may be the downside to having a large dungeon that you're dropped into rather suddenly, without time to prepare, that's also that difficult. Heck, a little direction would be nice, since I was complaining about the fights when I only had Riven and Irelia being too hard.
So, my suggestions; we need better armors available, enchantments are a little too expensive given the current drop rates for money (average drop for money on a per fight basis was about 30g by the end of the demo. If I could fight 100 fights and still not afford the items you expect me to have, it's time to do some tweaking). Maybe this means giving out more enchantments, allowing us to find better gear, etc. I don't know.
But now for the elephant in the room; I had a massive problem in this demo. When I finally beat the Watery Fiend for the first time, I ran into a rather... interesting problem. On all future fights against these 'boss monsters', I would suddenly find myself taking far more damage and dealing less than I did before. I don't know what just happened, but if this is some kind of monster scaling thing, it's not nice. I had the same problem with the stronger tentacle monsters in the caves; I beat one of them in the jumping room, but when I went for the second one to get some levels and gold, they're suddenly dealing ludicrous amounts of damage and tearing me apart. If we're intended to grind these monsters for cash and exp, this is a big problem.
And I was trying to grind. I would make sure to gain at least a few levels in every area, going out of my way to get into fights, repeat them as much as I can, fight all optional fights, etc. If I'm having trouble, I can't be the only one. Some tweaking of the monsters is required, even if it's just raising exp and gold or reducing exp needed to level, I don't know. But at every new dungeon and new turn, I felt extremely weak, feeble, and was struggling with even the most simple of monsters in every new area.