The 7D is such a big deal because anon has finally upgraded the AF system from a nine-point diamond with 10 hidden,invisible assist points to an AF system that can on-paper,compete with the 51-point AF system used in the professional D3 series, as well as the D700 and D300 and D300s.
The 7D is such a big deal because Canon has finally invented a color-sensitive light metering system, like the one Nikon invented in 1996. Canon has finally caught on to the idea of measuring RGB values to help its light metering system become "smart", the way Nikon has been doing since the 1990's.
The 7D is a big deal because instead of having a camera that loses out on paper to Nikon's semi-pro bodies (first the D200, then the D300) they have decided to do something much more than just iterate (the 10D begets the 20D, the 20D spawns the same old same old 30D, which gives rise to the virtually identical 40D, followed by the so-so sales loser the 50D). Canon has finally decided to really do some innovation at the pro-sumer/semi-pro level, instead of rehashing the same old body with a minor MP nudge for the past 5 years...
The 7D actually premiers some new technologies and some seriously semi-pro level specifications in a category where Nikon has really had Canon's number since the D200. The 7D is a lot more "camera" than the 20-30-40-50D revolving door models.