I've realized, I frequently am very vocal about things I don't like about EfU, which could lead to the idea that I don't actually like much of anything. Contrary to that, let me explain what I do enjoy-what gets done very well here.
Non-canon: The setting here refuses to be cobbled together from canon, rather than the nearest drow city being known, it is an unknown city. The Dark Lake is canon, but presented in its own manner. Famous NPCs other than Halaster never seem to make an appearance. It corrects the handicap of having to stick to "what is known" and the problem some players introduce by knowing everything "in the source books".
Gritty Survivalism: While the sense that Sanctuary could be destroyed at any time is admittedly disappearing, there is at least still the sense that the city could face forces that it can't over come and has to live with from Etorix to Traensyr. It is pleasant to see how Sanctuary has to navigate between these forces just to hang on sometimes.
Extend-a-Quest: These optional continuations for quests are awesome and an idea I fully intend to steal for my own uses elsewhere. I enjoy the extra challenges on Hoar, Clowns, Gnolls and have enjoyed them even when my characters wound up horribly and permanently killed trying to finish these options.
Death: EfU's death system makes sense. Players don't respawn and say, "Hey, Frank killed me. Can you execute him?" Players have developed the good sense to stay dead when they should and move on. There are always some snags, but overall, the system prevents those characters that hang around forever without doing something. Plus the route of retirement allows you to end a character by making a fantastic attempt to reach the surface. This is a good way to see characters go out with a bang, before they turn into a joke by plodding on long after they cease being interesting. The tool of leaving small reminders of certain characters after death/retirement helps ease the separation anxiety some likely feel.
Loot: The random loot has somehow made the marvelous balance of being interesting, without having to be horribly powerful. Use/day items are very rare, but you can find one use items that are relatively powerful. If you look, you can generally find an item that helps you get really good and fighting one kind of monster-but nothing that makes you good against everything.
Fed-Ex-Randomness: Fed-ex quests get stale and dull sometimes, but the inclusion of optional rewards can make them more like hunting Easter Eggs than walking from point A to B to get 50 gold. I love going from point A to B and getting anything from a Potion of Stone Skin to a Urine Soaked Rag. It makes Fed-Ex more entertaining at those low levels where you need it to accumulate some basic gear.
NPC Reactions: Its a joy to watch as the NPCs react to the changes in the world. Sometimes, dynamism gets mistaken for just PCs changing the game world, sometimes it is good to be reminded that NPCs may wish to change the world such as the current mercantile conflicts with Donrick/Macasta/etc and seeing the growing unrest with the canal Watchmen makes the world seem more alive than not.
Excitement without Epicness: EfU can make the defeat of a cunning goblin commander, ala Spitfire, feel like it is a major accomplishment. You don't need to stop a dragon once a week to feel like the players are saving the day. Most plots stay well below the epic feel, and maintain that sense of desperation and survival in the Underdark rather than the general "high fantasy" you find in most game worlds that just gets out of hand. "Oh, we saved the world again? Didn't we do that last month?"