I'm loving The Witness. It's a beautiful game, and challenges me in a way very different from just about any other puzzle game I can think of. I highly recommend it to anyone looking for a brain teaser and something off the beaten path of modern games.
Now, for those that do pick up the game, I wanted to offer some tips but can't say too much because the process of discovery is a major part of the game, so here you go.
First, PAY ATTENTION. Everything on the island is intentional and there are lots of things you could easily miss if you're not paying attention. Every intricate detail can be important, or at the very least, is worth a second look.
Second, IF YOU GET STUCK, MOVE ON. Don't go looking up hints/solutions online every time you're having some difficulties. Due to the way the game is laid out you will not have all the information to solve every puzzle as soon as you come upon them. Even if you do have all the information you may not know exactly how to put it together to form a solution. In these situations just take a break from that puzzle/area and move to the next. Come back later and try again.
Along those same lines, EXPLORE. There's much to do and see and learn by exploring and the beauty of the island only increases with every new discovery.
Lastly, ENJOY THE JOURNEY. There is an "end" in the conventional sense but in my opinion the majority of the fun comes from the journey itself. Take your time. Don't worry about what's next or what's at the end. Simply appreciate what is.