CDPR apparently aimed to unify the open world (i.e. Bethesda style) and story-driven (i.e. Bioware style) formats, by brute force. Actually, Bioware seems to have tried the same (and failed IMO) with Dragon Age: Inquisition. The Witcher 3 is definitely better than Inquisition because it succeeds in making side quests feel meaningful while blurring the lines between main and side quests.
And yet the main storyline is not remarkable, far too much time is spent on the Wild Goose Chase delaying the advancement of the main plot as it stacks the side-quests sky-high. As someone who has not read the books Ciri drops out of thin air and I don't think enough time was spent introducing her and her special relationship with Geralt as his ward.
There are serious problems all over the place. Level progression is very flat (1000 XP from level 1 to 2 and 2000 XP from 29 to 30) which may be a reason why there are balance issues. I very quickly out-levelled the suggested levels for the main quest (and stayed out-levelled) and consequently much of the main quest is far too easy. Ciri is ridiculously overpowered and her sequences seem pointless as a result. It's also very awkward to invest points in skills while at the same time limiting the active skills to just 12 at level 30. The game hamfistedly forces you not to be a jack of all trades in this way.
Additionally the alchemy system is dumbed down to the point that it utterly confused me. You only need to collect ingredients once, potions and bombs are then replenished when you meditate by using generic alcohol. It doesn't make any sense. Alchemy was wonderful in the first game, here it mostly isn't. This is bad.
The interface is also a mess: it takes one button to open the map, but 3xEsc to get back to the game. Excuse me? There is a whole layer of the interface, clearly meant for controllers, that is pointless on keyboard and mouse and only ever gets in the way. On the K+M interface this thing should have been a top or side bar, or simply not be there.
Then there is the sheathing of swords: combat on horseback has two competing things at work: you want to put distance between you and enemies so your horse doesn't panic, but too much distance and Geralt will sheathe his sword and mess up your next charge. Also you can't climb/vault when the game decides you're in combat. So if you end up in a canal during a fight, good luck getting out. If you run into a knee-high fence it'll likewise stop you as surely as a castle wall. You need to (also?) use the movement keys to line-up interaction, this will often cause you to light candles close to characters you want to talk to rather than start the conversation, or the candle that's often right next to a fast-travel marker.
What's very annoying is that many issues seem very fixable and simply a matter of more playtesting and balancing. Let none of this prevent you from playing the game though; it is absolutely worth the price.