Reading another thread made me realise one major fault of the combat design that goes a long way to explaining why a lot of people feel that the Prologue is unfairly difficult.
With no levels under your belt, Geralt can only block twice, but here's the crux: the same rules do not apply to enemies.
Enemies can block an infinite amount of blows without penalty.

I understand that CDPR were heavily influenced by Demon's Souls, but it would appear not heavily enough. In Demon's Souls, while blocking drains your endurance, it also drains enemy endurance, as does attacking. There's also a series of moves available to break blocks, and these are sufficiently signposted when used by players and enemies alike.
And crucially, magic is NOT linked to endurance in Demon's Souls, meaning if your block is broken and you have no endurance left, you're not completely defenseless.

It's like they got half way there, but didn't follow through.

Saying that though, I completed the game on hard and immediately started a new game to try different things, so I really do like the game, it just needs a little spit and polish :)

You do know that the enemies are NOT level scaled to your level. Right?

Blocking is for chumps. Dodge Baby, Dodge!

Or, you know. Quen.

I think there are narrative as well as game mechanical reasons for linking magic to endurance. Using too much magic WILL kill characters in this universe.

If i'm not mistaken, vigor is not the same as "mana." Vigor is a depiction of physical exhaustion, which can be caused by both casting signs and blocking. Plus, i'm pretty sure that Geralt rarely blocks, and tends to just get the hell out of the way. Meaning: when he DOES block, it probably exhausts him more than someone trained with a sword and shield. Not saying it's perfect gameplay wise, but it seems to fit the lore.

The real answer to this is the learning curve for the combat in prologue is steep. It's not hard, just a lot to learn quickly.

I played the prologue on insane and didn't die a single time. And only used Qun the entire time. The combat in this game is really easy once you get used to it.

In fact I'd say CDP has quite a project on their hands to make the combat at least 10x more difficult on hard setting.