I hadn't mentioned this particular love of mine before, but here it is, all laid out - E6 is a "version" of D&D 3.5, that changes the scale more than anything.
Regular D&D assumes that you start at level 1, and work your way up to level 20 and beyond - and, due to some shaky design, some classes start being a lot more powerful than others.
In E6, you have just six levels to make your name. In game terms, this means you're still mortal - no Genesis and private demi-planes and Wishing Genies to give you three more Wishes, etc. You're a guy who is phenomenal with a sword, compared to the man on the street, or who can throw a fireball that can wreck your house - but neither can kill a city. Well, not in one round, at least. But they are still heroes, legends amongst normal men.
After those six levels, your characters can still advance - through purchasing feats. One for every 5000XP above 6th level is the recommended rate, and the creator even encourages GMs to create feats that allow players to circumvent the level limits slightly - a feat to learn a handful of 4th-level spells, or to gain access to a class feature just outside the player's reach.
It also means that bad guys are a challenge from level one right up to the level cap - when a well-placed dagger can kill you at any level, you can't afford to be Great Cleave-ing your way through hordes of goblins, safe in the knowledge that your 30+AC will protect you. You can also use far more recognisable enemies - even Kobolds can pose a threat to unprepared 6th-Level characters. Orcs, Ogres, even Hobgoblins and Drow all scale well from low-level to the end, and Dragons remain top-tier combatants - anything above a CR10 Young Dragon is a major military-level threat, that a group of adventurers should not be facing unless it's with a hundred well-armed men at their back. And even then...
It helps keep magic items magical as well - a +1 sword is nothing to be sniffed at, when +3 items can't be crafted (unless it's by someone with god-like smithing skills), and true Gods would need to intervene to make a +5 weapon like a Holy Avenger. That's not something you roll up on a treasure table - that's a campaign goal. Masterwork gear becomes way more important, instead of just a platform for enchantments - it really does represent the best of the best, as it's nearly as good as magic weapons. Shit must be good. Plus, it encourages players to take more "flavourful" choices (like Wonderous Items) when they're spending their loot (as all the "best gear" is too high-requirement to be made).
With six levels, you also avoid the ridiculous splat bloat of 3.5 - no more "My character is a multiclass Cleric/Sorcerer/Fighter/Rainbow Servant/Tainted Scholar/Mystic Theurge/Eldritch Knight, watch him tear reality asunder with this combination of class features!". You get the main stuff that defines a class at 1st level, after all (like a spell book and funny hat, or Sneak Attack, or Rage), and everything after that is just gravy.
Classes become a lot more balanced - at 6th level, the Wizard is able to throw out a little bit more in the way of damage-dealing spells, and the Fighter is killing everything that doesn't have damage resistance. There's no monsters that require spells alone to kill, nothing that can throw Save-or-Suck spells at you with any great reliability, and the Fighter won't need to start blowing all his money on magical weapons to "keep up" with casters who will be leaving him in the dust with just a handful of utility spells (or replacing him outright with Summon Monster).
Yes, I know - it probably sounds vaguely similar to my love of Microlite20's stripping back of 3.5 D&D. But, M20 lacks depth - it's easy and fun, yes, and I'm sure you could end up running a full campaign with it, but it can feel somewhat "samey". Whereas, E6 has all the crunch of 3.5, with far fewer of the problems - like high-level play, people shooting for certain power-builds, the whole "Caster Supremacy" thing, whatever. It takes the best bit of D&D, the "gritty" low-level stuff, up to the heroic level, and makes it the game - excising the superheroic bullshit of later levels (or, indeed, the God-like levels for Clerics, Druids, and Wizards).
It's awesome. I thoroughly recommend it.