The Dryads in the Monster Manual are unplayable as characters out of the box. And this is actually for an entirely different reason than the reasons most vaguely humanoid monsters are unplayable - although most of those still apply. The issue with playing a Dryad is that every single one who doesn't specifically perform crazy shenanigans with permanent shrink item or obscure spells from the WotC website will die less than a day after leaving an area that isn't even the size of a large town, let alone going to another city or plane entirely. That doesn't mesh well with most adventuring lifestyles.