There are plenty of sites already out there listing all the many bugs or "features" that IE 8 has brought to the table but so far the two main ones I have come across are:
Problems when FirebugLite is also running in IE 8.
I was getting a "Function Expected" error on line 1499 in the IsArray function. As a workaround I added the following line as the first line in the function e.g
if(!_object) return false;
This prevents the error but the Firebug-lite console does not display correctly in IE 8 when the document mode is also IE 8. To get round this I set the document mode to IE 7 and then it works fine.
Remember IE 8 now has a console in the developer tools options for outputting custom debug messages plus its now got a great debugger tool with all the same features as Firebug so Firebug-Lite isn't really needed anymore. However it should still be possible to run the two side by side.
Issue with clip and rect function
I also found an "invalid argument" error when setting the clip properties using the rect function. The code was for a scroller and the original version that raised the error was:
this.canvas.style.clip = "rect(0 " + this.canvasWidth + " " + this.canvasHeight + " 0)";
The fix was to add "px" after the dimension values and separate the values with commas e.g
this.canvas.style.clip = "rect(0px, " + parseInt(this.canvasWidth) + "px, " + parseInt(this.canvasHeight) + "px, 0px)";
I have come across a few errors such as "null is null or not an object" (you don't say!) which have been simple fixes and have just required testing the values correctly before using them.
I am sure there are hundreds more errors and that many many hours will be devoted to fixing them all in a similar manner to the last 2 major releases of IE. However overall I am very happy that they seem to have caught up with Firefox and the others and have implemented a pretty decent developer tools section including a great debugger that actually gives you a useful error message for once. Plus I like the fact that the GUI hasn't changed very much in the way that IE 6 to IE 7 did and I do like the little touches like the accelerators.
It seems pretty likely that more and more companies will get to the point where developing for IE becomes too much work and stop supporting it. Market share of Firefox, Chrome, Safari and Opera are increasing all the time so it may not be that long until you find a lot of sites just redirecting to a Mozilla download page when you access their site in IE. If that does happen then it might be a shame to some people but Microsoft will only have themselves to blame and may find that they have missed a great opportunity with this release to prevent that from occurring.