She's one of the greatest female returners of all time. She creates angles on a grass court that no female (and to a degree, most male players) can even dream of playing on a clay court. To do those things with just solely power is impossible. The women's game lacks a lot of variety at the minute and a lot of it is purely power based, Serena is not one of those players.
Her main attribute is power but to say she has no finesse is wildly incorrect. Funnily enough her forehand that generates more power and pace than anything else, has gone to absolute shit these days.