It's one of my favorite Animes as well, and while the show is very different, I am still enjoying it.
The best way I can describe it is they "Americanized" the storytelling. The big bad guy shows up much earlier, Spike's background is foreshadowed (and straight up told) much earlier in the series, just in general the overall plot, and themes are much more spelled out and served to the viewer on a platter than the Anime.
Which, to the folks who consider themselves gatekeepers of "True Anime", I'm sure is heresy of the highest order.