The Javascript API is based on Dojo. ESRI has created some dijits for a map, and the basic pattern looks really similar to the Virtual Earth maps.

The Google Maps extended is just more javascript which extends the base GoogleMaps API to play nice with the ESRI services.

2D and 3D in Virtual Earth is also fully supported - again via javascript extensions.

Slightly different syntax across the APIs, but same basic patterns.

Embed maps and tasks into sites

Use ArcGIS Online or custom (ArcGIS Server) data sources.

Any supported projection. So you polar explorers are now in luck.

Javascript can be hosted by ESRI (like VE or Google Maps) and is in the Akami cloud (think fast, and globally cached - a nice touch).

Samples will be hosted in a live server. Much easier to dig into the samples because they are live. Can copy paste code into your html/aspx/??? file and start modifying. Very streamlined development cycle.

Examples working with a Dojox.grid (Nice because it takes a DIV as a target so the page layout is pretty smooth)

http://maps.esri.com/agsjs/surfaceprofile.html - surface profile demo

InfoWindow can contain whatever you want - i.e. another map

Symbology Model

Really nice demo of a symbol manager based on dojo controls. Incidentally it was running in FireFox on a Mac. Lots of options re: how client side features are displayed. Should be relatively easy to allow client symbolization. Again - looks very performant. They also have symbol sets. They will have a code-gen tool that will allow you to use this tool to setup your symbology, and then export the code to drop into your app. Also running in Safari. Only Solid_Fill supported in FireFox 2. All the saucy picture fills are supported in FireFox 3.

Again stressing the simple, but you'll need to have your CSS, HTML and Javascript chops in shape.

Virtual Earth Extender

Only Tiles - no dynamic layers, but all results can be sent back as VEShapes. Also an interative SDK. Looks just like the VE Interactive API. Javascript hosted by ESRI.

Google Maps Extender

Tiled layers or dynamic layers (via GGroundOverlay). The extender allows you to easily pump results back into GM. The Extender removes the need to convert between the native classes in the base API and the ESRI javascript API - i.e. you can pass a GM LatLongBound into a query. This should be a very smooth developer experience. 

Mapplets

Run within Google Map web site. Can be published to Google Maps. Pretty cool way to push stuff out.

Thoughts:

Again - this is really great stuff. Simple stuff is really easy, and I'm betting that the hard stuff is still difficult, but I'd wager it's miles better than the 9.2 ADF experience. We'll see what the 9.3 .NET ADF has in store at tomorrows session...

The information flow here is very important - design the application to show focused data in small chunks. This allows very small, fast transactions. That said, if you try to push tons of data into your browser, it will bog down.

Since the Javascript is hosted by ESRI, presumably anyone can just include it on a page. From there, you can consume the free ArcGIS Online services, and of course you can mash-in anything else you want. So it would see that there is a world of good free fun just waiting to be explored...