For me, 4AD's magic is in its elegant simplicity and its tweakability.

The elegant simplicity gets and keeps the player in the game. The first few sessions will require a bit of page-flipping (and/or appropriate judgement-calling), but it nonetheless flows quickly, with neither fuss nor muss.

Tweakability: after your first couple of delves you'll be adding your own classes, creating rules for managing the party outside of the dungeon (e.g. the "party stash" and whatnot), creating your own encounter charts, and possibly even your own dungeon map generators. There's nothing in the game which hinders players in this regard, and there's enough material in the system to provide a good basis of "what's really intended to fit" within the framework.

In the long term, i suspect that 4AD will be known not so much for what it is itself, but as a catalyst which will spawn many other similar systems, each tweaked for variable levels of complexity, depth, and play styles, but ultimately having roots which lead back to this game.

One play tip: stone or aluminum tiles used for flooring and walls, preferably in the size range 2cm to 1 inch or so, can be bought cheaply in large sheets at most hardware stores and make an excellent component for this game. See this photo album for examples of 4AD play using such tiles instead of drawn-on-paper dungeons: https://goo.gl/photos/ACpGecKRRvbpx2mJ7

(Pedantic note: the backslash-escaping of quotes in this review is not from me, but is a bug in the review tool.)