1. Black woman lead.
2. Features the gays prominently.
3. It's a Star Trek prequel (originally) that plays loose with continuity.
Those are the three reasons people hate it so passionately, with one or two switching places depending upon preference. Three only takes precedence (and in some cases 1 and 2 ignored completely) with the more
...special... Trek fan who MUST watch EVERYTHING and reference their complete set of The Official Star Trek Fact Files frequently throughout.
Everyone else just... doesn't watch what they don't like