I think the dryads should be sexy tree women, by default.
That's what they were in myth. Oak nymphs. That should be reflected in the game. Tree nymphs.
They should be magical and fey and secretive, hiding lore and generally being icons of that ol' Greek misogynistic "retreating beautiful woman of the wilds" image. They should care for children lost in the forest (like the Meliai cared for baby Zeus), they should befriend rangers (as they are Artemis's friend), they should die if their tree dies (like the hemadryads), they should be chased by amorous creatures (like Daphne fleeing from Apollo), they should be sacred to the nature gods, spirits of unseen power you must propiate or be struck down by.
Of course, a creature that hides most of the time, might occasionally help the party, and only fights them if they offend it (and even then probably in a sort of backhanded curs-ey sort of way) doesn't make for the most interesting combat. But D&D monsters should be about more than an interesting combat.
That said, they certainly *could* be interesting...teleporting between trees, using branches to scratch and spear, whirling plants around, even spontaneously growing things, or chucking seeds with magical effects, they could be wonderful little Controllers.
Now, if you want to take a raging shrubbery and call it some sort of "dryad" I really won't object. The mythopoetics are probably flexible enough for that. But you probably shouldn't call it simply "dryad." It can be a mutant, it can be a misnomer, it can be an ally, it can be a product, but it shouldn't be the creature.