Well... the short version is: there is nothing new in it. In medium version the new Internet Explorer 11 is an improvement from IE 10, but is still letting me down.
The install-process didn't go without hiccups. Naturally I didn't read any documents and my install halted on serial number requirement. In the Windows 8.1 preview: FAQ Microsoft provides the correct serial, though.
About the IE 11: It seems bit more robust than IE 10 which failed on trivial things if using IE 7 emulation. On IE 11 the developers tools are completely re-written. That's good news to me, who am a web developer. The problem seems to be, that they completely dropped emulation for previous versions. It is pretty much the way to go but ... why did they have to introduce such functionality in the first place?
I get it, browser race is on. It was on hold for 10 years, but in 2008 when Google entered the race with its Chrome, all the other players sure started putting some effort into their product. Which is a good thing. Microsoft's latest response is IE 11 and they simply have to drop the legacy and start shifting gears to catch up with the others. The real problem is that according to my logs, pretty much every IE-version is still in use. With IE 7-10 it was possible to state in HTML, that this site uses IE 7 rendering rules, or alternatively select a rendering mode manually. Now I cannot seem to find such a switch.
In conclusion: Windows 8.1 upgrade is a major letdown. The new "start" button does not deliver, I'm still using Classic Shell as a start button, IE 11 doesn't deliver. I'm sure I'll update, but it's nothing worth waiting for.