The rich APIs (either for flex, silverlight or javascript) should be able to consume, at least, non transactional OGC layers without development... I mean WFS, WMS, tiled WMS
Please add OGC WMS support to ArcGIS Online, and ArcGIS Explorer Online. This would make these tools very useful for organizations that do not have ArcGIS Server.
Because missing the support of WFS, we can not effectively promote the use of ESRI APIs to build webapps for organizations asking for Mashups with OGC compliant mapservices.
This point is very important for users in Europe where Inspire directive defines OGC as the standards for web mapping.
Regarding PostGIS support, non of the browser API's would be able to connect to a PostGIS database (security limitation). In many cases, due to cross domain policies, this wouldn't be possible without a proxy anyway. I believe that you can publish an ArcGIS Server service that pulls directly from the WMS/WMTS/WFS/WCS/etc, so the API's can talk to them via ArcGIS Server. The server would then simply be a 'proxy' for these services.
Btw. WMS and WMTS is natively supported by the client API's at this point.