I have nothing to add to the great advice you've had here, but just wanted to say that my 8 yr old GSD is the love of my life. Had him since he was a pup, he's the easiest dog ever, he just seems to know what to do before I tell him.
I did socialize him really well when he was young and I do think that helped a lot - so he's not fussed about any new situations, just takes it in his stride.
I would recommend dealing with puppy biting by 'yelping' loudly whenever they bite (basically mimicking how they themselves would communicate with other dogs if they were hurt). I did this whenever my dog nipped me when he was little, and as soon as I 'yelped' he would back off immediately.
He also got to the 'adolescent' stage where he tried to push the boundaries and become more dominant with me (I was a single woman, alone with him). Again, I would mimic how dogs play and wrestle with him, but always make sure I won. It really worked.
Things to look out for - well my dog recently had GDV, which GSDs are prone to. Frightened the hell out of me. He also now has weak joints, partly as a result of loss of fitness from the GDV episode.
Other things to look out for - being asked "Is that a German Shepherd or an Alsatian?" all the time, and also being told "Oh my dog hates Alsatians, he was attached by one once, he doesn't like the big ears.." usually by small terrier owners, who's dogs invariably attack my GSD while he looks at them and wonders whether he should bother squashing them with one paw or just walk away in an aloof manner....
GSDs, when brought up well, are the easiest, calmest, most wonderful dogs in the world. Beware you'll never want another breed.....