Because Flash Player 10.1 had introduced new API (mainly for handling touch interactions) that Flex SDK 4.5 had started to use.
For example, "flash.events.TransformGestureEvent" referenced in GroupBase.as
I think the question was "Which flex components will not work in right way if we compile project with Flex SDK 4.5 and then run it in Flash Player 10.0 enviroment?"
For example - as I understand some new components in Flex SDK 4.5 use flash.globalization classes - so new spark formatters and ?validators? will not work in in Flash Player 10.0.
Also we have to pay attension to 'string sorting' functionality - because it could be dependent on flash.globalization also.
Are there another 'danger' components in Flex SDK 4.5 ?
The most recent Flex SDK public build 4.5.18623 requires Flash Player 10.2 at minimum due to changes in Flash Text Engine,
there is no point to make Flex 4.5 SDK based application compatible with Flash Player 10 anymore
I just asked about this here too. Why isnt 10.2 the 'minumum required version' of the flash player from the flash builder app's point of view?
It seems the 'use minumum version required by the flex sdk' feature is broken inside flash builder.