Visited a couple of times while I was living in Moscow, Russia - would recommend to anyone. Especially if they like ouzo with every meal.
Greeks are a very friendly, loud people. Never shy away from asking locals for recommendations on things or for help with directions, they are happy to assist.
The public transit can be flaky in some places/times. Is the bus coming? If there's no one else at the bus stop, it isn't a good sign. But its easy to get rides from people or have them help get a taxi called.
I hope you like seafood or lamb. Personally, I ate a lot of moussaka, and was happy to do it. Ouzo was served with every meal past about 10:30am.
Lodging is fairly cheap if you're willing to stay in little hole in the wall kinds of places. There's about a zillion of them.
Kasseri cheese is one of the greatest cheeses ever created, do not pass it up while you're there. It's the best of the Greek cheeses, hands down.
If you're fond of the country of Turkey or Turkish people, keep it to yourself.
Greeks do a sideways head tilt thing to indicate "yes". They do a head and eyebrow raise to indicate "no". But they understand nods and shakes, too.
Visit Athens to see old historic sights and spend way too much money on souvenirs. Visit Thessaloniki for a more relaxed experience and less outlandish prices.
Develop a taste for ouzo before you get there. I cannot stress this enough.