If you really want the full experience, you need to play on the highest difficulty. I started on the easiest setting just to get the hang of things, but by the time I was in the last Act (there are 3) I was Death March all day, every day. You can increase the difficulty at anytime during the game in the options menu.
If not for that, you'll just hack and slash your way to the end. You'll barely have to use the signs, potions, or grasp the details of the combat on easy or normal. You're only getting half the game play like that. Which brings me to the question marks, you open new ones when you look at a notice board, and investigating those is well worth the time. Diagrams, potion ingredients, loot, money, xp, and places of power. Do the mastercraft missions for the Dwarf (amorer) at Crows Perch and the Elf (blacksmith) in Novigrad. They're the only ones that can work with the diagrams you get from treasure hunts.
Different merchants have different ingredients to sale. What you can't find at one, you might find at another. You can sell some crafting items for lots of money. Emeralds, Diamonds, and shit like that are worth lots of money and you don't use them for crafting too much, if at all.
You're gonna need this stuff if you crank up the difficulty. Be sure to buy all the treasure hunter maps from merchants that have them, they give you some of the best armor/weapons in the game. There's a potion in Novigrad that resets all your skill points, the shop above the barber. Be sure to check for all the free DLC they've dropped. Mostly aesthetic, but I think there are three armor sets you can get which might prove useful. Maybe one new mission in there as well.
I'm not bothering with a replay as I did everything already. The only question marks I got left are in Skellige. But for those of you that are going through a second time or just starting, heed the advice. You'll get tired of that long ass load screen at some point. And the only romances that matter are Yen and Trish and they both come with different endings. So pick who you want and stick with them is key. Romancing neither is an option too.
Also, invest in Axii, you'll be surprised how many fights you can avoid with this power. And poke your nose in people business at every chance. It's easy to miss out on some good moments of the game by neglecting peoples request or choosing to use violence to resolve things.
Missions only give you XP if you are within 5 levels of the recommended level. Once the level suggested turns grey, you only get 5xp or none at all.
Had to learn a lot of this the hard way, but hopefully it'll make your game go smoother.
