The Creation Kit can only be launched through Steam, which is mildly annoying, but not too bad considering Skyrim itself already has the same requirement. The new kit has some major new features mixed with scattered minor changes and improvements. Most obviously, the new Papyrus script has substantially more flexibility, with support for custom functions, loops, waits, multithreading and more. Quests have a new "Dialogue Views" tab with a branching visualization of the dialog. There are other changes related to AI, dialogue, quests, world editing, magic, characters (NPCs and creatures have been combined into "Actors" for example), scripting, and more.
Importantly, Bethesda has finally decided to fix the lip sync generation that they broke all the way back in Oblivion. The process was broken after an Oblivion Construction Set update and was never fixed. The process remained broken in Fallout 3's G.E.C.K. and the Fallout: New Vegas G.E.C.K. too. In fact the only way to generate the required files was to use the old Oblivion CS from before Bethesda broke it. The fix is long overdue, but appreciated.
The Steam Workshop looks mostly as I expected. The interface is good enough, but there seems to be very limited functionality (there isn't even a search function!) and some decisions destined to cause problems down the road. There is currently no method of manually downloading mods; you must "subscribe" to them. Your subscriptions will then automatically be downloaded, or automatically be updated if necessary. Sounds good on paper, but mods for Bethesda games are not always that simple. Automatic mandatory updates are fine for many mods; for many others, it may have serious negative consequences. Containers can be reset, updates can cause problems (or in rare cases irreversibly corrupt saves), not at all updates are always wanted (and there seems to be no way to add optional sub-files or anything like that), some updates require clean saves, etc.
Obviously, in most cases, mod authors will be aware of this and take appropriate precautions. But with the current set of features available on the Workshop, those precautions may be limited to uploading mods somewhere else. There is no support for optional files (except as entirely separate mods), no support for mod dependencies, no support for custom installation scripts, no support for conflict detection and/or resolution, and no support for asking users if they want to update or not. Those missing features wouldn't be an issue normally, but if you're trying to create a platform that automatically installs and updates mods for users, you're going to run into problems without those features. For example, even a "simple" mod like Lightweight Potions and Poisons has to direct users to SkyrimNexus if they want other weight options.
Despite all that, I will still probably be uploading any mods I create to both SkyrimNexus and the Steam Workshop when possible. Regardless of the problems, some of which will hopefully be addressed over time, the Workshop will be doing great work in expanding the mod user audience to people who have never even installed a mod before. The process may be messy but in the end a larger and broader mod community is a good result.
Of course, Valve being Valve, the very first mod available on the Steam Workshop was Fall of the Space Core, Vol. 1 created by "Aperture Laboratories."
"To celebrate the opening of the Steam Workshop for Skyrim, Valve and Bethesda have teamed up to bring you the Portal 2 Space Core mod, which will let the aggressively space-centric little robot tag along on your adventures in Tamriel.
Also, since Skyrim was the only major release of 2011 without Nolan North in it, you should consider this mod a patch to fix that problem. You can now feel free to include Skyrim in the "Nolan North" section of your video game library, which is to say, your video game library."
There are already some other great mods available: Midas Magic, Weapon Retexture Project, and even an early version of Open Cities. Right now, Midas Magic is the most popular mod (deservedly so) with over 3000 subscribers and 10000 views. There are already over 300 other mods available.
Finally, the "surprise" that Bethesda teased was of course the high-res texture pack of which evidence was discovered 1 hour after the "surprise" was mentioned. I haven't had the time to explore enough to see how good the pack is, but it is certainly an improvement. You can check out some comparison shots, and read a more detailed analysis (including some mistakes you can fix) of the texture pack in another thread created by Brumbek.
If you're interested in trying out the Creation Kit yourself, it is available in the "Tools" section in Steam. You may have to restart Steam for it to appear there. For information and help with the Creation Kit, you can use the Creation Kit Wiki or the official forum. Bethesda has also released three video tutorials explaining various concepts.
Finally, the "surprise" that Bethesda teased was of course the high-res texture pack of which evidence was discovered 1 hour after the "surprise" was mentioned. I haven't had the time to explore enough to see how good the pack is, but it is certainly an improvement. You can check out some comparison shots, and read a more detailed analysis (including some mistakes you can fix) of the texture pack in another thread created by Brumbek.
If you're interested in trying out the Creation Kit yourself, it is available in the "Tools" section in Steam. You may have to restart Steam for it to appear there. For information and help with the Creation Kit, you can use the Creation Kit Wiki or the official forum. Bethesda has also released three video tutorials explaining various concepts.