***early game spoilers below***
I bought KOTC earlier today and with a few hours in it, so far I'm nothing but impressed. Seeing a game with this scope and depth with only four people behind its creation is amazing. I have to admit I did not play much of the demo. I fought a few lizardmen and browsed the in-game documentation, but it was the design notes and rpg reviews on the website that really sold me. I enjoyed the discussion of what makes rpgs fun (and not), and I agreed with most of it. I told my one rpgaming buddy about the game and site immediately.
Speaking of in-game documentation, KOTC's hyperlinking help system is one of my favorite features so far. That I can click on a stat on the character sheet, or spell description, etc. and it leads me into the greater documentation is excellent. Every sufficiently complicated game should have such a system.
Of course, my other favorite aspect of the game, which it seems to be centered around, is the combat. I like that it is hard. I enjoy having to try different strategies/spells to succeed. The SPOILER: fight to save the usurper dwarf wizard Neptune was very interesting: surrounding him, making him invisible, etc. For reference I'm playing a mul knight/tank, half-elf knight/archer/dual-wielder, human cleric, half-elf wizard. I was level 5 when I beat the above dwarf fight.
Furthermore, there are a lot of details that are great as well. The dynamic icon+text tooltip is concise and helpful. I like the auto-filling favorite spell combat menu. The interface in general is fast and easy to use. Also, making all the spellcasting spontaneous and scroll-learned are nice touches.
Finally, purely in the interest of humbly helping to improve an already awesome game, here are a few nitpicks. (As of version 1.04)
1) Non-game: The download/payment section might could be made somewhat clearer. I'd never used paypal at all before, and the paypal button loaded an error page in Firefox (it worked fine in IE). Simply a couple lines like, "yes you can just pay with a credit card without having a paypal account" and "yes you can use your silly American dollars (at an exchange rate)" would have helped my thick self.
2) Interface: The numbers shown for saving throws on the character sheet and the corresponding readout should reflect the appropriate ability modifiers (DEX/CON/WIS). Most of the other data on the sheet list only the final modified value (which is very useful by the way!). Furthermore, saving throw buffs (e.g. Bless) do show up on the sheet and readout, so not showing the ability modifier seems like a minor inconsistency.
3) Interface: Non-magical masterwork equipment should be labeled as such, to distinguish them from the truly run-of-the-mill stuff. Simply "M. Longsword" would probably do just fine. This is really minor, but it saves a couple of clicks.
4) Interface: "M" should close the local map when it's open. Yeah I know, tiny (but helpful!). Similarly, "I" could close the inventory screen, etc. Also similarly, it'd be nice to be able to close the world map (with "W" and/or "ESC") without actually going anywhere.
5) Bug?: When I earned the campfire from the SPOILER: friendly bears in the SW cave by giving them zombie meat, I couldn't rest at it. I could click on it to rest, but my leader would stand on top of it, and be unable to rest. Exiting and re-entering the area fixed the campfire.
6) Bug?: When either of my casters exits a Silence spell's AOE, not all of their spells come back online during that round. My cleric for example, couldn't cast her summon elemental spells or Lesser Restoration, while the rest of her spells were castable. And the next round she could cast all her spells again.
EDIT: Figured this one out! Those spells require full-round action: can't normally cast after moving!
7) Typo: Magic stone spell description talks about "peebles", twice.
8) Typo: Ghoul Touch spell description "paralyse" hyperlink does not work.
That's all my tiny comments on the game for now. Thanks for reading them, and thanks for a great rpg! I may reply with more comments as I continue playing.