This is a great upgrade!
1) It is now possible to have dynamic expressions/graphics that can be changed in real-time by sliders, checkboxes, etc.
2) The graphics have been much improved, with hardware accelerated zoom and rotate, transparency and anti-aliasing. The default aesthetic algorithms are also much improved, so making multiple plots look good in a single graphic is much easier then ever.
3) Many obscure features that formerly belonged to packages have been included in the main kernel and improved. This includes graphics functions, like Arrow, and mathematical algorithms, e.g. combinatorics.
4) The documentation has been completely rewritten so that it is much more usable then before, approx 40,000 pages of good info.
5) The curated data is unmatched, over 500GB of data that can be accessed over the internet from within Mathematica. This includes data on stars, galaxies, molecules, countries, particles, and more!
There are so many reasons to upgrade, I recommend it highly. If you haven't looked at this page, check it out:
http://www.wolfram.com/products/mathematica/newin6/
This is what convinced me to buy, and I didn't feel misled at all. The features they talk about on this page are really there and easy to use!