We've been testing our first install of Server 2008 R2 this month, it will be replacing an aging webserver running Server 2003. IIS 7.5 brings with it a few new things, the most notable is a new interface. Less noticeable at first, is the lack of an SMTP server. Sometimes you need a quick and dirty SMTP server for delivering email from websites, and the Windows SMTP server is great for this. Coupled with the use of a smarthost for relaying, it can cover a lot of basic needs very well. Server 2008 R2 DOES actually include an SMTP server, but to access it you must install the SMTP components in the Server Manager, and then open up the IIS 6 manager (old school style), which allows you to manage the SMTP server. Now, strangely, the first few times I tried to access the IIS 6 manager, it wouldn't open. I received an error stating the wait operation had timed out. I checked everything I could, there was nothing useful in the event log... so eventually I removed IIS completely and then slowely re-added it piece by piece. Now everything works fine... though this morning I found the entire SMTP configuration had reset to default, so there may be something still a little wonky here.
I ran across another odd quirk, which seems to be default behaviour in IIS since 5.1, but I'm surprised that I've never encountered it before. The problem arose when we found Windows Authentication just wasn't working when we set a hostname. Without a hostname it would work, but with a hostname we just kept getting prompted for authentication over and over again. The event log revealed a login failure with the status code 0xC000006D. A quick Google search turned up this document which explains it: http://support.microsoft.com/kb/896861
So we disabled the DNS check, and now the sites all work fine. Strange things... but this is why you test.