I'm frustrated because I want to use Safari, but it is missing a few particular features that I can't live without. Firefox 3.2b has them; i am thinking, in particular:
greater multi-touch support (three finger swipe up or down to go to top or bottom of page, respectively).
when i close the browser, or quit, i like it when the previously opened tabs reappear in the new window. great for unexpected quits, restarts due to software updates, et cetera.
links will ALWAYS open in a new tab, never a new window. i know, it's possible by holding the command key, but i prefer not.
more to come, these are just my first thoughts. or hopefully someone can prove me wrong and say safari 4 IS capable of these.