Transacted NTFS
Perform file operations in a transaction. Haven't had the need myself but I can certainly imagine scenarios where this would be a great technology to have.

IIS7
The metabase is no more. The configuration for sites being hosted in IIS7 is done from the web.config. This makes deploying a site to a different server even easier than it is at the moment.
IIS7 divided into a great number of different modules. Only the modules that are required by the sites that are hosted on the server need to be loaded, so this provides a benefit to both performance and security. It is possible to create your own modules using the same interface as is being used by the IIS-team itself. (For the Apache-fans: yes, I know this is sort of like what Apache does already.)

Biztalk Futures
This wasn't mentioned in any of the keynotes, but in a breakout on the roadmap for Biztalk they demoed different improvements that are being developed. For anyone that uses Biztalk for real projects, these features will provide a huge improvement in productivity.

• When selecting a field or functoid in the mapper only the related nodes will be highlighted and automagically moved into the view.
• Finding the related field on the other side of the map. Select a field and hit shift; the mapper will suggest a corresponding field based on the name of the field.
• Relating port, channels, maps, etc.: they showed a single design-surface where all of the artificats are visible. This way the relation between these artificats becomes clear; no more swichting between different tools to setup a message transfer. This will make it a whole lot easier to 'grasp' an entire solution in a single glance.

This was one of the coolest demos I saw at this PDC (up to now at least..). There should also be a video on Channel9 on this subject. (Update: Although I haven't had a change to watch it myself, I reckon this should be it)

Both interoperability and extensibility have been the recurring themes during the last few days. Microsoft seems to be taking the critism that they haven't be known for 'open' systems and protocols to heart.
Indigo (aka WCF) supports the applicable WS-* standards; Vista is set to use RSS (anyone remember CDF?); Infocard supports X509 and everyone can issue their own tickets using the WS-specs on which Infocard relies.

All these developments continue to make this a great platform for developing solutions.