Worst change from the book: Making Quellcrist Falconer into a technophobe and the personal love interest of long-ago Kovacs. In the book she's a famous dead[1] figure from history who a) admirably fought on the losing side of a rebellion and b) wrote a lot of quotable things. She's like a cross between William Wallace and Sun Tzu. Falconer thought that her followers could play the long game against unjust power, since human immortality made time irrelevant; she didn't argue that immortality itself was wrong. In the series she's just that tired stock character, the pseudo-wise reactionary who argues that there are Things Man Was Not Meant to Know.
In the book Kovacs seems to love Falconer's ideas, but she lived before him and they weren't a romantic item. He's not interested in personally trying to fight a rebellion against the whole Protectorate, either. He's just looking out for his own skin and a few people he cares about.
Making Rei into Kovacs's sister was unnecessary and again went directly against the tone of the original: it made Kovacs sad and conflicted instead of furious and calculating when it comes to permanently stopping her.
The series is visually gorgeous, and if you haven't come into the series with the expectation that the same ideas and noirish conventions will be maintained from the book (as I did), you may well enjoy it better for that.
[1] Yes, I read the later Woken Furies where Falconer seems to "come back" in a way, but an event from decades after Altered Carbon shouldn't have been shoehorned into the Altered Carbon adaptation this way. Or at least I thought it shouldn't have been.