The difficulty is SUPER wonky, and not scaled to single players AT ALL in many instances. Any enemy who is a caster is about 10 times as dangerous and a non-caster. Bosses in +1 or more zones are virtually unkillable, and will often kill you in one hit. The upgrading system for weapons... doesn't seem to work. The unholy spirits, for instance, are supposed to give life on hit, but they don't. And I don't really see any benefits at all to using any of them, which makes me wonder if it's bugged.
I watched a stream all night of a guy playing with a group of about 8 people, and they are able to just graveyard zerg the impossibly difficult bosses down. He went from level 1-81 in a single day because being able to kill these really hard bosses nets you like 3 or 4 levels at a time, plus really good items. The fact that the difficulty doesn't seem to scale properly (at all?) with player count in the game really ruins it. Some things seem balanced for 4+ players, others seem balanced for single players, rather than dynamically scaling the difficulty based on the number of players in the game.
I still have not explored all the biomes, and while it's neat for a little while to find a new one (except ocean, fuck ocean) they don't feel sufficiently unique to warrant touting them as a major feature of the game. It's not like Terraria where the different biomes are completely different components of the game, with totally unique enemies, items, bosses and mechanics (i.e. hallow and corruption). And with reallly, really shitty biomes like the ocean which is just a completely empty body of water, they often feel like a negative, and I just threw a whole world away and started a new one because of it.
The loot progression is also completely one dimensional. It's got the D3 problem of bigger numbers, not mechanics. Again, comparing it to something like Terraria, you had all kinds of unique items in that game that would let you fly, or grapple around, or teleport back to town (REALLY FUCKING BADLY NEEDED IN THIS GAME). Here, you just make your numbers bigger. You don't get special effects on weapons or armor, you don't find special items that have singularly unique effects, you just get bigger numbers.
The skill trees are nonexistent. With 6 of the 9 skills per class being focused on adventuring and not combat, you don't get any level of customization, and the combat skills themselves don't progress in meaningful ways as you dump points into them, they just get lower cooldowns or grow by 1% movement speed per point. This is a problem with the touted "infinite level cap" where he has to scale everything off asymptotically or risk the game breaking. Unfortunately it just succeeds in making the skills boring once you've unlocked everything by around level 15.
Overall, I still played it all day, but it's a hollow kind of "all day". It's like an MMO, where I'll look back at the time spent playing it and say what the FUCK was I doing playing that shit all day long on the 4th of July? It benefits greatly from the party play and social aspect, but you have to get a bunch of like-minded people together who will be able to tolerate the boring MMO grind and the incredibly long and empty travel distances that bookend the interesting combat and exploration encounters in dungeons and missions.