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I see some meta-summaries here, but think that PoE actually has some important lessons for MMOs: it's an example of how you both modernize a game genre and keep the core essence of what people found compelling in the older games.
As background, this is a single player game designed to channel the spirit of the old Infinity Engine games (Icewind Dale, Baldurs Gate, et al.) It is a new system (not D&D), set in a late medieval setting - primitive firearms, crossbows, and traditional fantasy game armor / weapons as well.
You have a single character and will run with a party of six; you can either run with companions with story lines and quests or simply generate your own (silent) companions to your tastes. There are 11 classes. Some are familiar archetypes, although well designed (fighter, priest, mage, rogue, druid, barbarian, ranger, paladin); others are novel takes. The chanter is a very interesting twist on bards, with buffs developing as they are sung and powerful summons; the cipher is a psionic with unique resource mechanisms; the monk takes strength from getting damaged. All classes can wear all armor and use all weapons, so you can customize what they do. Characters are developed through talents (some available to all, others class-specific) plus some custom class spells or abilities. You have a mix of per encounter and per rest spell/abilties. Stats are different than the fantasy norm, and I found them an interesting twist: might, for example, boosts damage in general, and intelligence boosts range and duration of effects. So an intelligent barbarian with large AoE abilities is actually a good idea. Relative to classic D&D I think that character development is improved - without the same rigid class roles, but with interesting choices and a range of valid builds and approaches. Encounters are interesting, tactical and capture the core of the old IE games. There is danger even in routine ones, and if you play in a sloppy way you'll get slapped around. They also can play out radically differently with different party builds.
There are a lot of very clever improvements to the underlying game. One is that there is a temporary resource (endurance) drawn from a larger pool (health); if you drain endurance you're knocked out for the fight, but you're only knocked out for good (no resurrection) if you drain health. It works very well - allowing players to risk more dangerous encounters without save/reload. You can meter your resources out to delay saves or throw everything at the wall every time and rest a lot. They ditched the pointless tedium of encumbrance and inventory tetris with an unlimited stash. This is the sort of change that you only appreciate when you see it. You could eventually haul and store everything in the old games (stuffing junk into barrels, lugging it to and from dungeons); it just wasted a ton of your time. Now you can swap out different weapon sets, use different consumables, and so on; it's an enormous update. Money is a useful resource - not too scarce, but you won't be able to buy out all of the fancy goods in the stores. The stash system also makes crafting far less picky - you can make useful things without having to stash and retrieve piles of junk from containers. And the crafting is genuinely useful without being overpowered.
The storytelling is stellar. Modern games try to voice-act everything and have all sorts of cinematic flourishes. This is both enormously expensive and limiting. PoE tells a lot of stories with text, and they do it effectively. People used to cutscenes and people who click through dialog / ignore books, etc. are the ones who don't see what the fuss about the game is. It's a marvelously detailed world that deals with some surprisingly deep themes. The dialog is rich and you don't have everything telegraphed to you with a handful of obvious choices; act like a jerk and people elsewhere will treat you like a jerk. Play the game through with a different character and completely different options appear and disappear. You can avoid violence in a lot of places, or you can kill em all and let God sort em out. (There is no experience for kills, just for discovery and quests, so you aren't always best off mechanically slaughtering everything that moves.) This really opens up roleplaying opportunities.
For me, the take-away lessons are that you want to focus on the core appeal of older games (e.g. tight tactical encounters, character development, and solid lore / storytelling for games in this genre) while shedding mechanical limitations that encouraged tedium and hyper-conservative game play.