It looks like bashiok over on twitter replied about when we will get our surprise.
Random Map Generation.
Apoc has written a small article on map generation in diablo 3, expect to hear some more on this subject!
The "randomized" map generation has always been a defining feature of the Diablo series. In Diablo III, there's basically three types of layouts: Exact, Fixed border, and Variable.
Examples of the "Exact" would be places like New Tristram or the Defiled Crypt Level 2 where the layout will always be the same. "Fixed border" areas, like The Weeping Hollow, will always have the same shape and size, but the tiles inside can be arranged differently. "Variable" layouts are built by taking map tiles and connecting them together, which can produce unpredictable arrangements but still follow some consistency.
The selection of monsters for a given area is determined separately from the map generation, however there are tiles that can spawn monsters to supplement that. It is impossible to get an encounter that introduces a monster where they don't normally belong. For example, there is an encounter that makes 5 Skeletal Archers pop up if you loot a nearby corpse - this encounter can't appear on Cathedral Level 2 because Skeletal Archers don't appear until level 4. Encounters can also reward the player, so it might take a couple runs before you find that Resplendent Chest that always appears on the SW tile (hint, hint).
The Variable layout method is used in several maps in the beta including: Defiled Crypts, Cathedral Levels 1, 2, and 4, and the Den of the Fallen. It's the most interesting method because it's not as predictable as the other areas which can translate to a higher replay value. Cathedral Level 4 is a prime example since it's the single largest generated area in the beta.
The areas are composed of pre-generated tiles that snap to a grid. Like a puzzle, each tile has to be connected by at least one cardinal point side, where North is the top-right corner of the screen. Usually the tiles are connected seamlessly by doorways or bridges. To correlate with the lore, every floor's map must also have a meteor tile.
The map generation process follows a set of rules. Imagine starting with a blank grid. The formation of the map would then follow these steps for Cathedral Level 4:
1. setup a 3x3 tile grid, leaving out the center tile.
2. randomly pick the placement for an exit tile, entrance tile, meteor tile (m), and 2 extra tiles to any open edge. These tiles cannot replace the tiles in step 1 nor attach to themselves.
3. the entrance must always be placed as a West tile
4. if the meteor tile is placed on the outside of the ring in step 1, then one of the extra tiles in step 2 must be attached to its end.
5. draw in the tiles that are eligible for each spot on the above defined layout
(There are entrance tiles for Cathedral that face different directions, so it's possible the constraint of #3 is a bug)
And so we get something like:
This consistently produces a map of 14 tiles (or 13 if the meteor tile ends up in the center). The other levels of the Cathedral and Defiled Crypt use different algorithms and are about 1/4 the size.
It's likely that each level will have its own set of rules for generation, and the algorithm behind the present ones may yet change before release. I imagine it will become important to understand the generation process behind these maps, since it definitely makes players more efficient navigators and as a result more efficient farmers.
Force Strategy Gaming
and here are todays new beta screenshots!