Canon EOS 550D (Rebel T2i / Kiss X4 Digital) Improvements

Canon EOS 550D vs EOS 500D Key differences

• Higher resolution 18MP CMOS with gapless micro lenses
• ISO 6400 no longer in 'expanded' range (12,800 max remains the same)
• Redesigned buttons and new movie/live view button
• Customizable auto ISO ranges
• Improved 63 zone metering (iFCL)
• 3:2 format screen with more pixels
• Improved movie functionality
• Slightly higher burst shooting rate (though buffer holds fewer shots)
• HDMI control (CEC)
• SDXC compatible

This New camera is 18 mega pixels; equivalent to EOS 7D, which is good. LCD screen is now more wider 3:2 aspect ratio at 3" with improved resolution.

EOS 550D gets a full HD movie mode

• 1920 x 1080 at up to 30 fps, 720P at 60/50 fps,
  
• You also get full creative control (over apertures and shutter speeds, just like EOS 7D)
  
• External mic socket

63-point iFCL metering

• With this camera range we get metering boost with a new 63-zone dual layer iFCL metering sensor (first seen on the EOS 7D). The iFCL system uses focus, color and luminance information to determine consistently exposed shots. All focus points provide distance information to the metering system to determine proximity to the subject and allow the algorithm to weight the exposure accordingly.

Custom Auto ISO

• You can now select the maximum setting for Auto ISO (from 400 to 6400). Canon is obviously confident its new sensor and noise reduction have improved; ISO 6400 is no longer an 'expanded' setting (which is now reserved for ISO 12,800 only).

Remote

• Alongside the EOS 550D Canon launched a new slimline remote control, the RC-6, which can fire the shutter from up to five meters away (and allows you to choose immediate release or a 2 second delay, unlike the RC-5). The RC-6 is also compatible with the EOS 450D, 500D, 7D and 5D Mark II.

As per my understanding there are lot of improvements, but according to Pros usability these are very less usable. But its welcome move to everybody.