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 :)