Actually, IE8 follows the W3C standards to the letter (that's the ones that are ratified, not the proposed ones). It renders the acid2 test perfectly. The problem is that many sites are not constructed to the standards. Of all the sites I visit, I've only put three in compatibility mode* and in all cases it's just for minor text stuff that I could easily live with if I had too.
If Microsoft would force people to upgrade to IE8 or even IE7 these problems would be solved via the death of IE6, which is quite likely the cause of most compatibility problems on the web today (with IE7 causing the rest due to its incomplete CSS implementation).
*This does not include Microsoft's list of sites that are known to not work in standards mode and render in compatibility mode by default.