I don't know about it not being fun single player at this time...
I have been playing it for a while now, enjoying my singleplayer game while I wait for my friends to break though the inability to even make an account to buy the game phase...
So far I have cleared out 3 ruins, 5 castles and faced 2 giant monsters...
While some of its a grind, if you take a moment to appreciate what it is then its fun...
The ruins are boring in the way they are built though, needs some work there...
The Castles are interesting but once again it does need some work cause its quite easy to clear out after you work out the pattern, the bosses are fun to fight when they are melee but Magic and Range are too easy atm, even if your melee yourself, the amount of damage you do way counters theirs before you stun them and get time to heal up...
The Giant Monsters are the most likely reason the game can be give the "influenced by Monster Hunter" tag...
The Giant Monsters themselves are quite an interesting fight, they are large creatures that fill up at least 1/3 of the screen when fully zoomed out, they hit hard and can become quite difficult to constantly attack without dodging and moving out of the way of their attacks MH style and they actually deform the terrain around them as you fight... That Rock a nice place to heal up behind out of sight? THINK AGAIN, the monster just SMASHED THROUGH IT...
The item crafting pretty interesting when you get into the nitty gritty of it, playing any class which requires Iron is kinda annoying considering that the resource needed is only found in caves and unless you find mountains you will have a hard time finding them compared to the other classes. The weapons and armour themselves are quite well done and dispite my games always hitting me with the "Not your Class" Drops 9 times out of 10. The ability to find/buy the recipies to make better gear is a nice touch...
Adjusting the look of the weapons is fun too, though there are moments you wish you could just removed that block completely cause by the looks of things there is a cap on how many you can add, but I think thats stated somewhere on the weapon itself. Being able to increase the range of your melee weapon by lengthening the weapon is quite funny if you think about it, and having things like the boomerang and making it have a larger body allows it to hit a large area from what I have experienced...
While I can see it being more fun with friends, the game is totally fine to play singleplayer style. Moving around the area correctly though is a must, moving to each of the dungeons/quest areas is a must which is locatable by the crossed swords icon on the map screen...