The basic rules, basically, have you roll stats, pick a race, and pick a class. Feat-like benefits will be baked into the class reflecting an iconic archetype, and they might let you roll skill dice for ability score checks related to your class. This reminds me of a slightly less complicated 2nd Edition, as you could pick out proficiencies, so maybe it is more like 1st? Basic?
Going from basic to standard seems like a pretty small step. As I said last week, it reminds me of 3rd Edition's degree of complexity in that you get to pick skills, feats (or a specialty), and actual class features. So while you might like a mace-wielding cleric, you can break out of that mold. I am guessing that wizards will get to pick traditions, rogues schemes, and so on.
Advanced rules on the other hand open up a floodgate of possibilities, from ways to awarding XP, to hirelings and animal companions, to alternate magic systems, and more. He likens it to 3rd Edition's Unearthed Arcana, which is great because that is one of my favorite 3rd Edition books, stating that the game is designed from the ground up to encourage rules hacking.
I also liked that the rules are categorized according to how complicated they are to implement and use. Dials are fairly easy to implement (but can still impact a game quite a bit). The two examples were removing all magical healing, and awarding XP for stuff completing quests or the more nebulous "doing things related to your class". Modules do not change existing things, but just add more stuff to the game like henchmen and animal companions, as well as the oft-mentioned "tactical combat".
The more dials you spin and modules you add makes the game more complex, and will likely slow things down, but Mearls assures that you can go crazy with them without breaking your game. This is all well and good, but what about the third category?
These rules did not have a label, but unlike dials and modules changes the core of the game so deeply that perhaps they were never meant to be named. Unlike dials and modules, which play well together, Mearls warns that employing more than one "core" change requires some careful consideration, as they are designing them with the assumption that you are using one at a time. To delve further is to invite madness, or dig up a balrog. Something like that.
In all seriousness I am glad that Mearls is being open about this, and hope that like in Unearthed Arcana the final product includes sidebars that explain to you the potential ramifications that these changes can bring about (bonus points if they mention other sets of rules that work well/not at all with them).
The focus list at the end is full of awesome. With the exception of facing and hit locations I like all of it, in particular per-encounter resources, mechanical benefits for character motivation, action points, variant XP rules, fast-combat rules for hordes, magic item crafting, armor with damage reduction, variant magic systems, and horror/sanity.