I truly enjoyed Resident Evil Zero for the most part, especially with Rebecca as the protagonist. I thoroughly enjoyed her portrayal in Zero so much more than the RE1 Remake. I recently have been on a bit of a resident evil binge, having replayed a majority of the GC games (zero, remake, 2, 3, Code:X) and I have to say that Rebecca is up there as one of my favorite characters. I replayed Zero before the Remake, and after going through Chris' Scenario I was pretty shocked at how dull I found Rebecca's portrayal in the remake was. She mentions nothing regarding what she discovered during zero, along with her journey to the mansion, or her experiences dealing with it all. There were a bunch of little details that irked me, for example in zero she is unable to play the piano well enough to open the passage in the bar, but in the remake she is capable (albeit with some practice) of playing a relatively difficult piece on the piano. The change in voice actresses to me was very noticeable; and I wished they'd just stuck with the voice actress from resident evil 0, who I found best suited for Rebecca. Apart from these minor issues, Rebecca is still one of my favorite characters in RE.
As for things like the enemies, while I personally can see why they were what they were (pretty much all B.O.W experiments in a facility designed for testing on animals/insects/etc.) I felt a lack of interest in some of the bosses, both in design appeal and difficulty. Many of them can be killed without taking any damage, for instance the scorpion on the train can be killed with only a knife and the giant centipede if you just stay out of the path it runs around. For the most part, I felt normal monsters such as the Eliminator Apes and the Mimicry Marcus' more challenging and scary than the bosses. The bosses were no doubt not one of the strong points of the game.
All in all I think zero is one of the best re titles. I really hope that we get to see a return of Rebecca and Billy in future installments of RE.