Apologizing in advance for what will be a long post based on my limited experience with KI:
What does it have to offer that MKX does not? Fantastic online, multi-layered mind-games with the combo breaker/counter breaker system, consistent anti-airs, fairly simple combo system with a surprising amount of depth after you've learned it, ULTRAAAAAAAA COMBOOOOOS.
What is the Practice mode like? (is there hitboxvisual, Frame data ECT.)? Hitbox viewer can be turned on in practice mode to show you your character's normal hitbox, throw hitbox, and hurtboxes on all moves. Frame data is available on the command list for specials and command attacks, not sure if it's available for light/med/heavy strikes, I haven't looked that far into it.
What is the Cast like? I only really have experience with the season 1 characters, but they all seem very solid. Each cast member has one specific "trait" unique to them (i.e. Jago can link doubles in combos into doubles of lower strength, and when he completes a full cycle he gains a small amount of instinct meter, or Fulgore's reactor spins faster/builds meter faster after linking auto doubles in combos) and it can really add a lot to your game. The season 2 characters all seem to have an extra resource to manage in some respect (except for Kan-Ra if I remember correctly), or some quirky trait.
What is Block like , specific? Back to block, down+back to crouch block.
Is footsies a big deal? As in any FG, footsies is huge, especially here since a mistake can lead to a 60%+ combo if your opponent has the resources. Those combos can be broken if you know the opponent's moveset and can break with the button of corresponding strength, but if you make a wild (wrong) guess or mistime you're locked out of a break for at least 3 seconds.
What is online like? The netcode is godlike. I have never played any game, much less a fighting game, with such good online. I've only been playing for about 2 weeks, and only in short bursts online, but I've had great experiences without all of the salt that usually comes from winning in an NRS or Capcom game.
is it balanced? I can't speak to that. I picked the game up shortly after watching it at EVO, and there's no specific character that has vexed me yet. I know that Thunder is pretty strong and Sabrewulf has no bad matchups. That's about it for my knowledge of the game's balance so far.