What we know about Windows 8 is incomplete and unofficial - garnered from job postings, rumours and the slides allegedly leaked in June 2010 by a software engineer at HP responsible for OEM relations.
The slides include plenty of marketing ideas rather than technical details, they show that Microsoft has its eye on what Apple is doing to make its operating systems so popular and they declare themselves a work in progress.
Not only is every page marked 'this is not a plan of record' but the opening discussion includes the line "reality: there are currently more ideas than there is time to implement them". That's especially true if the Windows 8 release date is as soon as we think it might be.

Windows 8 release date
Windows 8, say the slides, will be available "for the holiday" - but not which one.
There's a timeline that doesn't have many dates - the one suggesting that the coding would begin in June 2010 is suspect when some sources say the M1 (milestone 1 build) is already done and there's what we assume is a typo that we'd correct to say the third Forum (rather than the second) would be in July. (There are several points where the slides are incomplete or confusing; for instance a pointed reference to "creating great Dell + Windows Experiences" in a deck that otherwise tips the hat - and appears to have been intended for - HP.)
The timeline put the first beta of IE9 in August 2010, along with the shipping date for Windows Live Wave 4 which fits other rumours and positions them just after the third Forum.
UPDATE: The first beta of IE9 arrived in September 2010 and Windows Live Wave 4 was released in June 2010.
That makes the forums three-to five months apart; assuming an average of four months - and assuming the chart is to scale and that the dates don't slip - that puts Windows 8 beta release date a little before March 2011 and Windows 8 RTM shortly after July 2011 (a date suggested on the blog of a now-ex Microsoft employee which you can find preserved, with the boxed version following in autumn 2011 - for the holiday).
We've said before that we expect Windows 8 release date to be in early 2012 and we don't expect Microsoft to talk about a date until the Milestone 3 build, which would be around November 2010 by these calculations.
UPDATE: Milestone 3 came a little later than we expected: WinRumors reported on 1 Mar 2011 that the first Windows 8 Milestone 3 build has been compiled by Microsoft. We're not expecting a Windows 8 beta release date some time in summer 2011.
There are several statistics in the leaked slides (typical RAM, network connected TVs, mobile broadband penetration and 4G deployment) that talk about the specs that will be common - in 2012.
Interestingly, the timeline shows Windows Live Wave 5 with a short development cycle that finishes before Windows 8; that matches suggestions that Live will offer more cloud services for Windows 8.
UPDATE: On 25 October 2010, Microsoft Netherlands said "it will take about two years before 'Windows 8' [is] on the market."
UPDATE: On February 21 2011, ZDNet published a slide showing the internal Windows 8 roadmap. Given leaked information on some sites around current build information, the author of the ZDNet piece surmises that Windows 8 will see a mid-2010 RTM.

Windows 8 system requirements
Both Windows Vista and Windows 7 have system requirements of a 1GHz processor and 1GB of RAM. Vista requires 15GB of free hard drive space, and Windows 7 requires 16GB.
Given that there's been no real jump in requirements from Windows Vista to 7 (unlike the jump from XP to Vista, where XP required a 233MHz processor and 64MB of RAM) we'd expect Windows 8 to happily run on a system that can run Windows 7.

Windows 8 price
Windows 7 Home Premium costs £99 for an upgrade copy and £149 for the full version. Expect the Windows 8 price to be similar.
Windows 8 interface

Although no final Windows 8 screenshots have surfaced (it's too early for those), on 17 March 2011, we reported that Windows 8 could offer a cut-down version of its user interface, taking on some design elements from the Windows Phone 7 UI in the form of Aero Lite.
Windows 8 features

The 'fundamentals' Microsoft is aiming for with Windows 8 include "a fast on/off experience, responsiveness, and a great level of reliability from the start".
The 'big three' are boot time, shutdown time and battery life ("Windows 8 PCs turn on fast, nearly instantly in some cases, and are ready to work without any long or unexpected delays") but Microsoft is also thinking about how long it takes to get things done - how long until you read your first email, see the home page in your browser or start playing media. PCs should feel like an appliance that's ready to use as soon as you turn on the power.

Windows 8 startup
FASTER STARTUP: Windows 8 will show you what slows down startup and if removing an app you don't use improves it

Mobile PCs should resume 'instantly' from sleep (in under a second from S3 sleep), and booting up will be faster because of caching, with a boot layout prefetcher and the ReadyBoost cache persisting even when you reboot.
As only 9% of people currently use hibernate (which will work more quickly in Windows 8 because system information will be saved and compressed in parallel), Windows 8 will have a new Logoff and Hibernate combination that closes your apps like shutting the PC down does and refreshes your desktop like restarting does, but actually caches drivers, system services, devices and much of the core system the way hibernation does.
Turning the PC back on will take about half the time a cold boot needs (and the slides point out that on many PCs the power-on tests take longer than the Windows startup, so BIOS makers need to shape up).
It will be the default option but it won't be called Logoff and Hibernate; Microsoft is debating terms like Shutdown, Turn Off, Power Down and thinking through how the other options for turning the PC off will show up in the interface.
You'll be able to use an encrypting hard drive to boot Windows 8 and they'll integrate with BitLocker and third-party security apps.
Improving battery life will be based on some deep changes to the kernel; removing an interrupt in the kernel scheduler completely and removing more of the timers that interrupt Windows when it's trying to save power.
Windows 8 might get the same option for powering down unused areas of memory to save power that's on the cards for Windows Server, it will block disk reads and writes and some CPU access when you're not doing anything on your PC and PCI devices can turn off completely when they're not in use (assuming the drivers for specific devices support it).
Windows 7 stopped laptops waking up automatically when they're not plugged in; Windows 8 will get a new 'intelligent alarm' that can wake them up for things like virus scans, but only if they're plugged in.
OEMs will get new test tools that check the performance, reliability, security and Windows Logo compatibility of the PC, as well as measuring performance in Outlook and IE. And depending on whether partners have "concerns" about it, Microsoft might give the same tools to journalists, IT pros and users.

Windows 8 multimedia
Windows 8 will have better media playback and recording, but it will balance using hardware acceleration to save battery life and using the CPU when it gives a better result.
Audio will use hardware acceleration more because that does improve battery life. There will be post-processing to take out blur, noise and shakey video filmed on a phone or webcam, and support for more codecs including AVC and as-yet-undetermined 3D video codecs (stereoscopic3D support is coming, for games and for 3D movies in Media Center, but there are format issues).
Microsoft talks about sharing 'with nearby devices'; one way that will work is adding the Play To option currently in Windows Media Player to the browser for HTML 5 audio and video content, so you can play it on any device that supports DLNA, another is APIs to let other software do the same.
That will work with DRM content, if it's protected with DTCP-IP (digital transmission content protection over IP) or Microsoft's own PlayReady and hardware acceleration will speed up DRM decoding.
There's also a new 'remote display' option that will let you send your screen from a laptop to a large monitor (which will use DirectX hardware acceleration and the same multimonitor interface that's already in Windows 7, but for wireless displays as well, which could be an Internet-connected TV - Microsoft refers to 35% of TVs having network connectivity by 2012 and wonders whether to prioritise Internet TV over further improvements to broadcast TV).

Windows 8 Help and Support
In Windows XP the Help and Support centre was a branded hub of tools and links; in Windows 7 it's far more minimal. Windows 8 will go back to the branded experience, with integrated search for support forums run by your PC manufacturer but add the Windows 7 troubleshooters.
It will also link better with the Action Center, with tools that show more clearly what's happening on your PC; what apps are running, what resources are being used (like Task Manager showing which apps are using the most network bandwidth), how and when things have changed and what they can do about it. It will also include an Application Management tool that will let you find what apps are causing performance problems and adjust or remove them.

Windows 8 task manager
IMPROVED TASK MANAGER: Task manager will make it easier to see why an app might not be performing; here the Zune software is using all the network bandwidth to download podcasts, so video in the browser keeps pausing. We hope the white on black isn't the final design!

The Windows pre-boot recovery environment will be simpler, combining the safe mode and 'last known good' options into one interface. It will use what Microsoft calls 'superboot' to remove malware and rootkits
If you have to reset your PC, Windows 8 will restore "all the files settings and even the applications" although you'll have to go to the Windows Store to download apps and get a list of apps that didn't come from the store, so it's not clear how automatic this will actually be.