I went to Thassos 18 months ago. It was great, I loved it and would go back again. I stayed at the Alexandra Beach Hotel in Potos, nice hotel and lovely little town with a good range of bars and restaurants. Of course it's just a very quiet island so I don't think you'll find any noisy nightlife but the bars were fun and traditional rather than too tacky. It's a bit of a pain to get to with having to get the ferry over from the mainland but that all added to the holiday for me. We hired a car for a few days (much cheaper if you hire in the town rather than through your tour company). We went all round the island and into the hilly interior, we pretty much saw everything in 3 days so probably no need to hire a car for longer than that. Assuming you like traditional Greek islands, I think you'll like it!
Went to Thassos a few years a few years ago - very quiet and very Greek full of fishing villages and top beaches. Its very green and thats because it does rain a little bit more than the other islands