I mean "trap." Guardian Druids don't have the HP or the Defenses to make secondary defenders, because nearly all of the things you can do to boost defenses don't work in Beast Form. Shields, Hafted Defense, Rhythm Blade, etc., the only thing that works is Hide Armor Spec. So you're a low hp, low defense secondary Defender with a poor Mark punishment. And of all the people I've seen play Druids, almost none of them respected the Beast Form restriction, which might explain why anyone thinks they're good.
Remember: you can't use any utility powers without the Beast Form keyword, either, it isn't just attacks. And the best utility powers, for the most part, don't have the Beast Form keyword. So just stay in caster form all the time? Doesn't work, the best attack powers (and the best controller powers) have the Beast Form keyword, particularly among the encounter powers. The dailies are about 50/50. But if you're meleeing in Beast Form, that shift out isn't always enough to get away to use a Ranged caster power,
Swarm Druid? Poor riders, and that resist is completely insufficient after the HAE nerf, particularly with MM3 monsters. The rounds to die calculator just laughs at Swarm Druids at this point.
Predator Druids aren't God awful, but their control is still sub-par, and their striking, even thoroughly optimized, barely does 2/3 of a normal striker's damage, and in order to pull it off they give up basically all control and have to expose themselves to getting punched in the face.
Short: If you want to control the battlefield, play a Wizard or Invoker. If you want to be a Defender who does damage or completely locks down a section of the fight, play a Fighter. If you want versatility, play a Paladin|Warlock, Swordmage|Wizard, or hell, play a Fighter.
There is no one thing the Druid that does that other classes do not do extraordinarily better and they are, on average, so poor that picking a class who is baseline really good at one thing and that specializing in another direction will result in a character very good at both those things at the same time. Druids can be good at one thing at a time (literally, if they're in Beast Form they completely lose being good at whatever their Caster Form is doing and vice versa), and not that good at it, because Beast Form is a trap.