The next update will fill in the worldgen sites for everyone. This means you will be able to find and explore worldgen mountain fortresses (dwarves), dark fortresses (goblin), forest retreats (elves), and inhabited caves (kobolds), along with other settlements. I've included the site names rather than attaching "vanilla" race names to them because you can mod in new races to use those.
Armies will move around the world map now (I think) and you'll have army camps to explore in adventure mode (I am certain of this). Sneaking has been implemented. You'll have some hero roles to play as the adventurer, and I think we can have non-lethal fights now, which may improve tantrum spirals considerably in dwarf fortress mode with extra violence.
We have multi-tile trees (both above and underground) as a result of the new elf sites, and climbing has been implemented.
You'll also have succession in play. This means if individuals holding positions in a civilisation die, they will get replaced even if they're not the diplomat or outpost liason, so you will get a new monarch, goblins will find new leaders, etc. Worldgen demons may be able to take over civilisations whilst your fortress is running. I hope this update will also fix the bug with the outpost liason being replaced after leaving your map, and make grass on mountains regrow properly.
Aren't siegers supposed to tunnel in the next update?
Its planned, but will probably be an init option so you can switch it off. I don't think its going to happen for this release, as we're about to hit the wrap-up stage to check the new features that have been added.