So! Next update is here, and it's a pretty big one.
First off...Power Stars. Every once in a while, your miners or gatherers will uncover star pieces, and Toads can now create a Star Beacon, which allows them to fuse the star pieces into Power Stars. The Star Beacon can produce a number of effects, and the more Power Stars you gather, the stronger your fort will become. Power Stars are not used up in these reactions, only broken into their component Star Pieces, and most of the reactions affect your entire fort... for a whole year. Mind, it'll take a while to collect enough for the really big effects... the highest level power requires 77(!) of them. You can also collect Power Stars in other ways: by defeating megabeasts or by raising Lumas to maturity (this can only be done on Good-aligned mountains where Star Bits are found). Only Toads can utilize Stars in this manner, although Koopas and Shy Guys can still use them to increase the value of their fort...or melt them down and produce powerful weapons. (Toads can do this too, but as a melted star can't be restored, it probably isn't worth it.)
For the Koopas, metal-based pets and creatures now have a new use: you can butcher them and melt down their shells for extra metal. Tap-taps generally produce 2 copper bars, Buzzy Beetle shells produce 1 bronze bar, and Chomps can produce up to nearly 20 bars of iron at full size (assuming you want to slaughter your Chomp, that is). They can also make a new ranged weapon, the Bullet Bill Blaster. It's heavy and fires slow but deals great damage and knockback.
Shy Guys can now make bonfires. Just 'cause. And since they're already a kind of cheat mode as far as fortress-building is concerned, if the game gets boring you can now up the ante by producing useless, but absurdly valuable, Subcon Crystals. That should bring on the sieges quick enough!
Speaking of sieges, the adventure-only races are now a lot more interesting. Each of them has something unique to offer as an adventurer, and their attacks on fortresses are more threatening than before. Frog People now produce poison like their wild Frogog cousins, Mole People have extravision and claws like daggers, Bub-ulbs can spread sleep- and dizziness-inducing pollen, and Bumpties will resist blunt attacks while they gradually penguin-slap their enemies to death.
Plant and fungus creatures now have more realistic and detailed anatomy. Bean-based creatures have a tough seed-coat, for instance, and most plant creatures now have edible parts. And before you ask, a hymenium is the part of the mushroom underneath the cap, where the spores are held. They function as brains and can be gouged. Also, plants, animals, and fungi each have different syndrome classes now, so certain creatures will be immune to diseases that others are not (for instance, Toads are immune to most venomous bites and stings, but are susceptible to the Blorbs and transformations.)
All the civs are more segregated in worldgen and tend to build in specific biomes. So when you're wandering the wilds and run into creatures like Beanies and the like, you'll know that the town over the horizon is probably Beanish, and if you're in a barren wasteland you're more likely to find Koopa fortresses. (The only problem with this system is that sometimes civs tend to wind up 'trapped' by unfavorable regions, preventing their spread, and can sometimes leave vast sections of the planet uninhabited, depending on the world's shape. Usually all of the major races end up on one continent, leaving the smaller continents to around one to four races...which isn't such a great loss, seeing as how there are only four main races in DF to begin with.) Humans have more of a presence now, by the way. They'll tend to build their major kingdoms in deserts but can also show up elsewhere from time to time.
And lots of new creatures, of course - enough to make each biome different, and each game a new experience. Most of the new creatures are from the RPGs, though there are a few surprises too.
Only one new secret this time around - it's called the Star Beam (AKA Smite Evil). Now the night will know why it fears you. (It works on demons too!)
Have fun!