I know this is in the damage forum, but we totally agree about block. It has not kept up as a mitigation stat.
I also agree Armor Pen is confusing. You can't look at a piece with Armor Pen vs. say AP and have a feel for whether it's an upgrade or not. If you see haste or crit, you have some idea of what that will do for you. Armor Pen may just be damage in a slightly confusing package.
Long-term we could totally see merging MP5 and Spirit into one stat (with all of the class changes that would entail) but we're not ready to pull the trigger on anything like that yet.
Source
5/22/2009
Block failing, mp5-vs-spirit
A little something for each paladin spec here. More on block falling by the wayside, Armor Pen sounding like it won't change for a while, and mp5-vs-spirit seems to be at a forefront but they aren't ready to commit to a large change. I really hope they don't change pally/shaman to use spirit like druid/priest; it's nice to have some distinct classes.