Grinding Gear Games developed their own engine that works well but that's not the important thing. What really matters is the artistic aspect of the game. When you get into dungeons, or cathedrals, or temples and so on, all levels have been designed with a maniacal care for details and some awesome art such as mosaics, paintings and so on. It is simply amusing to walk through randomly generated maps that give such fine compositions. It is very important to take the time to level up at the beginning of the game. Whenever you need to buy chaos orbs, you can turn to U4GM for help. We has cheap poe items for sale.
Anyways, Lots of art can be found in any other aspects, from the passive skill tree screen, to cards, or items you will find all the time. Definitely good talents have been working on this game.
Soundtarck. Themes and Accompaniments.
Yes, soundtrack is incredible too. I can't believe how inspiring and epic it can get, with most of the title tracks (changed through time, from expansion pack to pack), and other themes you can hear along dungeons or various locations. They all fit the atmosphere with music mostly made by arcs, choirs and other particular instruments such as sitar. This convinced me to purchase the soundtrack as well and this happens rarely.
Pros
1. FREE. Honestly, this is the very first reason you must love and worship this game and their developers. This title is simply put "well developed" in all its aspect and it's immense. It's not a pay to win and you will end up buying stuff on your own will to support developers because they did an immense job and definitely deserve it.
2. SUPPORT. I started playing this game in 2014 when it barely had 3 acts, now in 2017 it has 10 acts and an infinite map system that expands the game further. How can you hate a game that constantly expands... For free?
3. DIABLO ON STEROIDS. If you love Diablo and other isometric ARPG this is the definitive game for you. Base mechanics are always the same, but here everything is expanded way beyond your imagination. If you love text books with stories and lore, here you can find books as well around the maps, not to mention all discussions with NPCs in towns.
4. COOP. There's only one thing that makes this game even better: coop. You can play all the story with your friends, make a guild and go around. You can also challenge other players, there are public events, giving even more variety to the multiplayer and social aspect of the game, but I didn't try them as I only care about coop.