Closed web clients are sexy:

Enhancements to the Drawing API make runtime drawing much easier with re-styleable properties, 3D APIs, and a new way of drawing sophisticated shapes without having to code them line by line.

Your video card can be used to do compositing on all raster content. Utilizing the hardware processing power of the graphics card, GPU compositing accelerates compositing calculations of bitmaps, filters, blend modes, and video overlays faster than would be performed in software on the CPU.

As a developer Flash 10 looks extremely compelling. You get a platform that just works across the 3 most used operating systems, a very powerful drawing engine optimized for speed and what is probably the most comprehensive runtime environment outside of native applications.

The advantage for closed web clients is clear: the company controlling them has absolute power over them, and can expand and change them in ways a consortium or an industry design committee would never dare.