Post here things that you came around to knowing later than you would have liked, and a hint built in to the game somewhere could have helped. For me: You can click on the "table" background and drag the entire screen. (The first time I needed this was when I went to MP for the first time and my team was "empty" and off to the side, which meant mostly off my screen. I stretched to cross my browser into my second monitor to get around this. Far later I accidentally click-dragged the background and realized what I was supposed to have done). You can hire more than 3 party members at a time (I knew you could hire more people at the Tavern. The tutorial told you to go back to hire others. But I thought I had to delete someone before/when I hire a new guy, losing my levels/progress on that character. I only found out you could hold more than 3 when I decided I REALLY hated having a slow dwarven warrior and was willing to re-gain my levels). Now... my first one MIGHT be explained when I get to the MP tutorial. I haven't got that far yet, and went to MP on my own (something which maybe should be blocked. Supposedly you don't have to be TOO high level to trigger the MP tutorial, but I basically gave up on the campaign for a while after finding MP and enjoying running around with full gear).
There are the game hints to help with some things, like control-clicking. It's just that hint implementation has been nebulous. If you'd like to read them all, look here: http://wiki.cardhuntria.com/wiki/Hints
One thing I am still not quite certain of, but pretty sure about: You cannot have the same card attached twice. I have multiple Righteous Frenzy in my Cleric's deck, but trying to cast 2 in a row on my warrior doesn't give him a +4. However, if I activate Unholy Frenzy + Righteous Frenzy, those two will stack for +4 that turn.
Units can only have three cards attached at a time. If you add a fourth the oldest (leftmost) is discarded. So if you're encumbered you can clear it with any combination of three (traits, buffs, DoTs like ember spray etc)
que!? is that so? this really should have been mentioned somewhere in the hints. or was it? I vaguely recall a case where I wanted to move (or something) one of my guys with winds of war (or something), but the issue was that my wizard only saw one enemy and the game did not even ask me who I wanted to move, but chose the default option (that one enemy) without giving me the choice to target my ally. maybe it was not winds of war (can't remember!!) but the situation, which I'd like to mention here, was that at one time the default "target the one enemy you see and no choice given" worked against the crafty ally-targeting I was intending to pull off. (Maybe it was that spell attack that knocks the target backwards, whatsitsname! and I wanted to knock my own guy a square further. but instead it was just cast on the only eligible enemy target.) (I'm not even sure that this is a problem.) (that's feedback for you)
On the winds of war point, I personally just turned of auto target in the options menu, so it highlights all options that I can choose when I use a card.
My enemy just hated learning this one: Elvish Mobility works for enemy elves too (he had one, I had 3... THANK YOU!)
It wasn't obvious until it happened the first time that it was OK for one (or two) characters to die in a scenario.
Things I learned today: How your username is capitalized is based on how you type it when you log in, not when you create your account. The biggest dungeons in the game (Garnet Portal, Batford's Manor) have 5 items in the last chest instead of 4.
I'm still surprised by just how uniquely different party makeups play out the adventures. You may start with 1 warrior, 1 priest, and 1 wizard; but altering that group can be a lot of fun.
One thing I kinda understood, but didn't really digest the importance of, was that you cannot get experience for your party on missions too far below your level. And you have to re-gear yourself to be of an appropriate level for things well below yourself (a pain once you start using power tokens). But a brand new party with all your nice gear can BLAZE through those low level missions. It is quite well worth your time to level up alternative parties just to farm drops off of the lower level dungeons when you have time to play, but not really enough time for the drawn out multiple map fights that show up in later levels.
Last night I sold everything I owned and bought a couple hundred Simple and Ornate chests. And I did not get a single treasure. (I didn't get a single legendary, either.)