What's new / changed (key points)

21.1 megapixel full frame CMOS sensor
The EOS 5D Mark II delivers an 8.3 megapixel jump in pixel count from the original 5D. This new sensor is said to be based on that of the EOS-1Ds Mark III (indeed it has exactly the same pixel count) but has several small changes, the hint being that it's actually slightly better.

DIGIC IV image processor, 14-bit ADC
The 5D Mark II becomes the second Canon DSLR (after the EOS 50D) to feature the new DIGIC 4 processor. This enables various new image processing features (such as lens peripheral illumination correction) as well as dealing with 21 megapixels of data at up to 3.9 frames per second (or 82 megapixels a second). It also enables video capture although we're told a second chip does the actual encoding.

Reduced micro lens gap
Canon have stated that the gap between microlenses has been reduced compared to the original EOS 5D, although there is still some gap (the EOS 50D has a 'gapless' design). We are guessing that the gap has been maintained to enable a shifted pattern near the corners of the frame (which helps reduce fall-off on such large sensors).

 Expanded ISO sensitivity range
Both the EOS 5D and EOS-1Ds Mark III have a 'calibrated range' of ISO 100 to 1600 with expansion of ISO 50 to 3200. Despite the smaller pixel pitch the 5D Mark II pushes the sensitivity envelope considerably further providing a calibrated range of 100 to 6400 with expansion of ISO 50 to ISO 25600. It will be interesting to see how it performs at the highest sensitivities.

Auto ISO in all modes except manual
The Mark II now features an automatic ISO option where the camera selects the sensitivity (in the range ISO 100 - 3200). In Auto, Program and Aperture Priority the camera attempts to maintain a minimum shutter speed of 1/focal length (hence with a 24 mm lens it will try to maintain 1/25 sec or faster). In manual mode Auto ISO is fixed at ISO 400.

 No mirror flip in Live View with Contrast AF
If you use contrast-detect AF (called 'live mode' by Canon) in Live View the Mark II doesn't need to drop the mirror between exposures, this means the lag between pressing the shutter release button and the exposure is much shorter (it also means that in this mode camera is metering using the main sensor). This also makes the exposure much quieter.

EOS Integrated Cleaning System
Now standard across the entire EOS range it wasn't that surprising to see Canon's EOS Integrated Cleaning System on the Mark II, including the new fluorine coating see on the EOS 50D. This will be a very welcome change for owners of the original 5D, which has become somewhat infamous for its ability to accumulate dust on the sensor.
Continuous shooting up to 3.9 fps
Being conservative (or perhaps bitten by previous claims) the Mark II has a specified continuous shooting rate of 3.9 frames per second. It can shoot a burst of up to 78 JPEG frames onto a normal CF card or up to 310 using a high-speed UDMA card.

Live view functionality with contrast-detect AF
As is the fashion these days the EOS 5D Mark II has Live view, and of course it has both passive AF and contrast-detect AF support. But most importantly it also has face detect AF, which we're confident will be perhaps the most frequently used feature on this camera.
Movie recording in live view
Perhaps one of the biggest 'news' items for the Mark II is the provision of movie recording in live view. You can define the size (1920 x 1080 or VGA) however other settings (such as frame rate; 30 fps and compression level) are fixed. The Mark II will record a movie clip for a maximum of 12 minutes at 1920 x 1080 and up to 24 minutes at VGA. Movies are recorded in Quicktime MOV format using H.264 codec for video and PCM for audio. At 1920 x 1080 the bitrate is 38.6 Mbits/sec (about 4.8 MBytes/sec) and at 640 x 480 the bitrate is 17.3 Mbits/sec (about 2.2 MBytes/sec).

Clear View 3.0" LCD monitor
When we first saw the Nikon D3 / D300 and Sony DSLR-A700 we were blown away by the VGA resolution screen, it wasn't much of a surprise to see this same screen make its way on to the EOS 50D and now the EOS 5D Mark II. Canon do seem to have made it even better with improved viewing angles and no less than three layers of anti-reflective material.
Automatic LCD brightness control
Something unique (among Canon DSLRs) to the EOS 5D Mark II is a passive light detector which when enabled can automatically adjust the brightness of the LCD monitor to make the screen easier to see outdoors in bright light. However in use it doesn't always work perfectly, which can make images look darker on screen than they actually are. So if you are going to use this feature, then it is a good idea to keep the histogram displayed at all times.

Built-in Microphone and Speaker
To support its video capabilities the EOS 5D Mark II has sprouted a microphone (front of the camera just below the 5D logo) and a speaker (rear of the camera to the right of the viewfinder). In addition there's also a microphone socket and AV output (see below). The microphone can only be used to record sound during videos. It cannot be used to add audio annotations to images (a feature in 1 series cameras).
Microphone input socket
Again in support of its new movie capabilities the 5D Mark II has a microphone input socket, as well as providing audio output via its A/V socket. The built in microphone is not very good in noisy environments, and sound quality is not fantastic overall. An external mic is required if sound is important.

HDMI output
Again another 'me too' feature is HDMI output, which can be used for still and movie playback but doesn't provide audio for movies.
Command Dial positions
The Mark II now has three more positions; two custom (C2 & C3) as well as the 'Creative Automatic' mode which is a carry over from the 50D and we wouldn't expect to make a big impact on the average 5D Mark II owner.
 98% coverage viewfinder
Viewfinder frame coverage has increased two percentage points to 98% (over the EOS 5D) which is very good but has already been eclipsed by the frankly huge, bright view provided by the 100% coverage, higher magnification viewfinder on the Sony DSLR-A900.
IrPort remote
The new IrPort remote sensor provides InfraRed remote control using Canon's optional RC1 or RC5 remotes.
 All new 1800 mAh battery
The LP-E6 is the same size as the old BP batteries but provides 1800 mAh of capacity as well as more detailed charge and life information, it also features recessed power terminals perhaps to avoid accidental shorting and appease the various travel authorities. You'll probably also need that extra power to shoot movies.