Same with Gen III, it just didn;t have all the old Pokemon before tha national dex - only Gold and Silver implemented all the Pokemon up until that point. Gen IV actually did it very similarly to Gen IV. In general, Gen IV always seemed like kind of a rehash of Gen III for me.

I'[m not a big fan of Gen IV just due to how it didn't feel it brought too much new stuff to the franchise. Gen V is my favorite because, aside from the brilliant story, it actually tried to shake some things up, and I loved that you were forced to use only new Pokemon. That way you couldn't just rely on old favorites - you had to familiarize yourself with the new guys and I think that's part of the reason why I like the Gen V Pokemon so much. I hadn;t seen them before and it all felt as magical as first encountering all those new guys in Pokemon Red and Blue for the first time.

With the darker atmosphere, a deconsturction (and reconstruction!) of the Pokemon formula, on top of generally trying to add so many new stuff to a series that many people felt was starting to get stale, I just really liked Black and White overall.

I also don't hate the designs at all. Gen V easily has my favorite bunch of Pokemon in the whole series, but that's always very subjective