Reading the original IE Blog Article and the /. Discussion on the X-UA-Compatible markings, I have reached a couple of conclusions.

• There is a camp of people who think that standards are an end to themselves free from nitty gritty details like solving real world problems.
• Some people understand the issues but are happy to give the finger to billions of lines of working HTML pages and HTML generating code because it didn’t have the honor of being standards compliant years ago when it was written. (I think these people are secretly interested in donating their time to a  Y2K style effort of fixing all these old sites)
• Many commenters don’t understand the constraints of this particular problem

For the sake of the last camp, I will attempt to make the issues clear up the problem constraints (and fail).

1. It is unacceptable to break existing pages. If a person’s favorite site doesn’t work, they will avoid the upgrade or downgrade back to the old browser.  Assuming that all browser upgrades brings us closer to interoperable web standards, non adoption of the latest browser version is a very bad thing.
2. Most existing content is immutable. There is too much of it and too much work to fix all the HTML and HTML generators which originally produced it.
3. Web Standards and Implementations are not instantaneously mature, which means that all implementations will ship with bugs. While this is painfully obvious in IE, it is also demonstrated elsewhere: Firefox 2 doesn’t pass ACID 2, Firefox 3 will. What happens to pages depending on the bugs in Firefox 2 fixed in Firefox 3?
4. There is an awful mess of pages out there that will forever be in Quirks mode and IE6/7 "standards" mode. Any solution that doesn’t deal this issue is broken.
5. It can’t be a one time fix (debatable?). Something like a one time doctype change is not sustainable and leads to the same problem over and over again (since there will always be the latest new standard). Any solution should be good to handle this type of problem again and again for every browser vendor.

So now that you understand the constraints and you still have issues, make the world a better place and figure out a better solution then Microsoft did.