Well, a lot of the protection type breeds need a LOT of training. An untrained German shepherd can be a nightmare (I live next door to two that bark non-stop at us) they are also prone to temperament problems (shyness, fear biting etc)

If you like the look of the GSD, consider a few similar breeds.. the Belgian malinois, Belgian sheepdog, and Belgian tervuren.. as well as the Shiloh shepherd.

I can't say enough good things about dobermans.. I have never met one I didn't like. One of my friends owns one, he is the BEST trained dog I know. They're super sensitive to their owners' wishes and just really sweet dogs.

I personally do not trust rottweilers. If you got one definitely get it from a good reputable breeder of show/working dogs. I was attacked by one when I was 4 (not harmed but it sure was scary having a 100lb dog knock you over and pin you down!) and when I was older they had them at the barn I rode at and they'd try to corner you and growl at you. Also, one of my co-workers was bitten on the neck by one (I work at a grooming salon) ..she has a good scar from that one. We've had several aggressive rotties come in.

Another option is the various mastiff breeds. Definitely low key dogs, but they are devoted guardians. The old English mastiff (aka just mastiff) is known as the 'gamekeepers night dog'. They instinctively know (properly cultivated with socialization and training) just how much is needed to avert an attacker. They are the sweetest (and biggest) dogs out there. English mastiffs and bullmastiffs are the most common, however dogue de bordeaux (French mastiff) are seen more often nowdays as well

Farm guardian breeds like the great Pyrenees are a good choice too (lots of hair though)

That'll give you a few ideas.