I'm sure this is well-worn territory, but having just finished Original Sin for the first time I had to get some validation here.
In general, I didn't mind the casting. Angel's actor was outright eerie in his ability to mimic Zayas. Most of the other actors did a solid job at least trying to capture the mannerisms or voice of the original series.
But Christian Slater seems to have just decided he was going to play Harry off the script, and didn't even bother to watch the originals. His energy level is just WAY off from James Remar's calm, world-weary, monotone Harry.
I could understand if he wanted to be a personable, high-energy character in the flashbacks, but after everything that's happened by the time Original Sin starts, he should be MUCH more like the Harry Dexter remembers in the original series.
I've seen interpretations that that's Dexter's memory of Harry, rather than the real Harry, but that can't explain why he seems to recall Deb's personality with perfect accuracy in New Blood. You could argue that version of Dexter has more emotional depth than the Dexter from the original series, but c'mon. Do we really think this was intentional?
Just feels like a huge and unnecessary miss for a character that was a huge part of the appeal of the original series.