Chris Perkins on the rate of advancement: power level is increasing (although not by a "startling" amount); 20th level in 4th Edition is a little more powerful than 20th level in 3rd Edition. However, you'll reach 30th level in the same time you used to reach 20th level --
"The way character advancement works now, it takes fewer encounters to gain a level, but it takes roughly the same length of time to reach 30 levels in 4th Edition as it takes to reach 20 levels in 3rd Edition. The rate of level advancement is still being playtested, however, so the jury's still out on whether the final game will work this way.
One of the goals of 4th Edition is to make high-level play as fun, balanced, and manageable as low-level play, and to make high-level characters as easy to create and run as low-level ones. Comparing high-level 4th Edition characters to high-level 3rd Edition characters is not an apples-to-apples comparison because they're built very differently. However, there isn't a startling increase in overall power level from a 20th-level 3rd Edition character to a 20th-level 4th Edition character "