Silverlight and Pluggable Codecs
Flash has been doing video on the web for years now, and doing an awesome job at it. The Flash Player has a number of built-in codecs, and all the Flash video on the web plays in the latest Flash player, no hassles. And HD videos look great.
Microsoft, on the other hand, has their various video container formats (ASF, WMV, AVI) where the codec that you need to play the content is dependent on the streams in the containers. In other words, given a *.avi file, you have no idea what you need to have installed to play it.
This leads to the various codec packs you can download, which almost invariably are a vehicle for malware. It's a terrible situation, and I think one of the reasons Flash video has done so well.
Microsoft apparently doesn't get this, because the latest version of Silverlight has "Extensible Media Format Support", which means developers can plug in arbitrary video or audio codecs to support other video formats.
Argh.
So one site will be posting Ogg-contained videos and another DivX videos, and I'm going to need to download a bunch of codecs to view videos with Silverlight? No thanks.