We've heard for a long time that big companies won't upgrade from IE6 because some crucial legacy app was built specifically for IE6. Unfortunately the definition of "App", weather its a web app, desktop app, browser plugin(!), or something else is not specified.
But what actually breaks when upgrading beyond IE6? I can't really think of anything since the engines are similar, but I'm obviously wrong.
+10 if you can point me to such an app.
I don't think there are actually any major changes between the two. For example, by supporting IE7 jQuery is essentially supporting IE6 because they are so similar. But IE7 apparently did introduce a couple new bugs (unfortunately, I can't tell you what they are) that were not found in IE6.
Relevant:
Q: The need to support Internet Explorer 6 and some of those older browsers, how much of that is holding back jQuery, and one day when you can get away from that, how much more capability do you think it will bring to jQuery?
John Resig: If we dropped support for IE6 today, absolutely nothing would change, at least for jQuery core. jQuery UI might be able to benefit more. The JavaScript engine and the DOM engine in their export did not change between 6 and 7. That is a bit of a lie. They did change. They introduced two bugs in IE7. It’s really frustrating. In our case, it would be better to drop IE7 support than IE6. The reality is that from my perspective, IE6 is the same as IE7. IE7 is shipping in IE8. IE8 is going to be around for a very long time. There is no reason to drop any of these.