Good idea for a thread.
Here are some of my own points:
1) I really like W3. The main quests, side-quests and contracts were stellar. A++. However I felt that the scavenger hunts and random map markers were very "video-game-y". The main quest, side-quests and contracts made me feel like I was Geralt, but the scavenger hunts and random map markers made me feel like I was playing a video-game where the objective was to go to point a, get object b and get result c. I know it's a matter of man power and there are only so many hours in the day, but when every scavenger hunt is the same chest and every map marker is either a guarded treasure, bandits or a lost chest it really gets old after a while. It makes it feel like a video game and not an RPG, it break the immersion. In fact I would even prefer that there were MUCH fewer random map markers in W3 if they couldn't be made more individual and original.
2) I didn't particularly care for the mutagen and slot system in W3. It felt like you were always short on slots and instead of feeling like a bad ass with all those cool powers you unlock it felt like I was barely keeping pace with the game world and enemies. It felt like the system was handicapping your build rather than helping you out, if I had 6 free slots it felt very inefficient to combine 2 red and 4 blue powers. The best done leveling systems make you look at it and go "hell yea, I'm going to kick some ass". W3 mutagen and slot system always made me look at in and go "it's a mess, where shall I put that yellow power, I really wanted that delusion sign skill but it won't fit, sigh I guess it will have to do". I'm also not sure that being able to change your build so easily adds to the game, on the one hand you can change your build. On the other hand trying, and committing to, another kind of build is one of the best reasons there are for a second and third playthrough.
3) Which brings me to my third point. At higher difficulty levels, and in NG+ in particular, Witcher 3 is severely unbalanced, bordering on broken. I have always liked sign builds, in W1 you could kick ass through the entire game with a sign build. In W2 you could kick ass the entire game long with a sign build. In W3 I started out with a sign build and finished the main story with it, even though the last two boss fights were pretty difficult. Then I tried HoS with my sign build and hooolllleeee heeelll was it frustrating. There are some bosses in HoS which are pretty much immune to all signs, regenerate health and require sword damage to beat. Sure, this might be in keeping with the witcher lore and so forth. But if you give players the option to do a sign build I think you should be consequent and make that a viable playstyle the entire game through. In DM NG+ there is only one build that really works in my experience and that is lots of red combined with green synergy, decotions and quen. Everything else is bordering on masochistic. This is bad game design IMHO. Different builds and strategies should be viable even as the game progresses and the player reaches higher level. It's a bit like if you played through the entire Baldur's Gate 2 and then in the expansion you discover that the only build that really works is a fighter-mage hybrid class.
4) The world beyond the trodden path. The world in W3 is amazing, the mountains, the forests, the rivers.... the small bridge across the lake, the ruins in the forest, the ice on the lake.... I could go on forever. You have some very talented world designers on your team. However, it all feels pretty empty as long as you move outside the scope of a quest. In Skyrim you can wander off and get sidetracked for hours and hours, finding interesting ruins, people, books, lairs and weapons and armor as you go. In W3 when you wander off the beaten path the game feels pretty empty, the feeling you get is "damn, this setting is great they should have done something with it. How about a gwent playing troll under the bridge or SOMETHING". Some of my fondest memories from Skyrim were when I discovered a completely new and previously unknown dungeon or quest simply by exploring. I rarely got that in W3, and when I did the payoff was rarely that great. Don't be afraid to make big side quest and hide them from us. We will find them, we will play them, we will experience them, we will go crazy on the internet over them and we will love you for them. Don't try to make everything so streamlined, every player doesn't need to find or see everything on their first playthrough, it's ok.
5) Anything else than the custom armors and weapons in W3 felt pretty underwhelming. Not once did I find a set of armor, or a sword, which I wanted to replace my Griffin set with. After about 2/3 of the game I didn't even look at the stats anymore, I just kept the expensive ones and sold them. This was made worse by the fact that you are handed these supposedly legendary weapons in the story, like the swords you get from Hattori or Crach. Garbage both of them, sold within 5 minutes. Well - I kept Winter's Blade in my chest because it was a family heirloom and a gift, but you get the idea. Please don't do this. Having loot and supposedly epic weapons be so underwhelming can be pretty immersion breaking. The best designed weapons and armor in other RPGs have unique traits that force you to make trade-offs based on your playstyle and other gear. And no - having a different crit change doesn't count as unique, sorry. Look at all the weapons in Baldur's Gate 2 for instance, like the "flail of ages" or "spider's bane". Those are unique weapons with unique traits that really makes the player care.
I'm sure I could come up with more but I think those are some of my biggest points right now.
