I run a small firm specializing in web-based business applications, so I thought I'd offer some counter-points.
20 character limit, exporting, and sharing
It's true that is isn't a matter of just adding on another hard-drive. However, even with everything else factored in storage is cheap. One of the sites my company designed and is maintaining is one which allows ranchers and feedlots to upload post-mortem photos of cattle for veterinary review. We're seeing a large numbers of photos being uploaded per day (one recent tweak was to allow users to queue more than 100 photos for upload per batch), and with the different resized and tiled version of the photos, each takes up about 2-3 MB at the low end. I'm paying about 15 cents per GB per month. Start with characters in XML format taking up about 100K, then figure out how many characters would fit in that 15 cents worth of space. Sorry, I'm not buying the storage cost argument, not a bit.
Yeah, HTML 5 is not quite ready for prime-time. IE9's wide adoption may help with this, but that's not today. However, you can come up with some very user-friendly interfaces using HTML4 and Javascript.
Silverlight is simply a poor choice if your goal is "ultimate portability." Consider the large and growing number of environments, not just Windows and OSX, but the Linux, the iPad, Android devices including the upcoming Android tablets, the upcoming Blackberry tablet (based on what they're calling Blackberry Tablet OS), the upcoming WebOS-based HP tablets, and whatever else is coming along.
But don't take my word for it. In a recent blog post, Bob Muglia, President of the Server and Tools Division at Microsoft, stated that while they were committed to Silverlight as a platform for in-house corporate apps as well as Windows based mobile device apps, maintaining a runtime across all the available devices was "practically impossible." "We think HTML will provide the broadest, cross-platform reach across all these devices."As for leveraging the team's .NET skills, .NET languages can be easily used on the server-side for web-application development.
I think character builder is just the tip of an iceberg.
I'm going to judge the character builder on what it is or isn't now, not on what it may or may not be at some unspecified point in the future. And while I'm going to wait and see what the next couple months brings, some of what I'm seeing and hearing aren't exactly filling me with the warm glow of hope.