I can verify that the ones posted by "unique_username" are accurate!
(quote) Actually a TON of stuff has changed.
First off, it REALLY matters what mode you are in. In IE8, there are 3 (THREE) modes.
Now, if you are rendering in IE5/IE7 mode, then Nothing changes except that there will be a few methods added that shouldn't interfere with your page.
However, if like the majority, you are running with a doctype set, and thus in IE8 Standards mode, the following changes have occurred.
1.) document.getElementById( id ); //now only returns the CORRECT matches!
.getElementsByName( name ); //now only returns the CORRECT matches! nope, not fixed!
3.) .getAttribute( name ); //returns the CORRECT value for the given attribute! 4.) .setAttribute( name, value ); //actually SETS the name/value CORRECTLY (no more cAmElCaSe crud)! 5.) CSS Expressions are NO LONGER allowed (deprecated) 6.) Operation Aborted errors will still be thrown (in some cases) however the cases are fewer, and the error won't kill the entire page/DOM 7.) The attributes array on elements should (from the RC build onwards) be correct in terms of contents, have a length, etc. 8.) Button elements now submit the contents of the value attribute, NOT the HTML contents of the Button Tag
There has also been a bunch of CSS 2.1 fixes, so things that rendered weird before, or needed hacks, should be much better. (see below for details on alpha/transparency - there's been big changes)
See the IE Blog for details.
Also see Web Bug Track for fine grained details on Bugs, Fixes for IE8 (and all other browsers)
SVG, rounded corners, ECMAScript based Event Listeners, Better Form Element design/events etc. are still missing.
PS If you have specific issues, let us know and we'll help hunt them down for you. ;-)
window.resize events are currently broken in IE8 Beta2 and IE8 Partner Release 1 (will not fire) now fixed in RTM build
window.open(); in IE8 Partner Release is sometimes failing "claiming" that the target url is not available (quirky, hard to reproduce)