As a DM of almost three decades, and several setting/rules revisions, I was drawn to this project due to the stated backwards compatibility with 3.5 rulebooks. It is trumpeted as a refrain in all of the press releases and promotional material.
There are many things that seem to require as much conversion as 4e seems to present. The races and classes are fine examples of this. I want a Pathfinder wizard to be able to adventure side by side with my Gatecrasher (PrC from the Manual of the Planes) without massive bookkeeping or in play balance issues. In the curent Alpha form I do not see this.
Skills are another area where this is prevalent, and one that I heartily disagree with (see my posts on the Keep Skill Points thread).
There is also a lot to love about the fixes you are looking at: perception is wonderful, the CMB for resolving combat maneuvers looks great and will be playtested next week in my game.
I would like to invite the other playtesters and forum contributors to share their thoughts here on the subject of compatibility. I have over $3K in 3.0/3.5 books, and I am sure I am not the only one.
George "Loki" Wiliams
http//planejammer.wordpress.com
http://www.planewalker.com
http://www.spelljammer.org