I have gotten a lot of experience with Plone this year. I am currently launching the 5 company websites on Plone this month.....and have had our company intranet on plone for more than a year.
You have a lot of options in plone with most of what you need working out of the box.
All of your requirements above are met by plone. 10k sessions a day....you just need to make sure you have enough RAM. But other than that performance is pretty rockin.
The nice thing is you can download and run plone on your desktop to check it out in about 20 minutes.
Runs on windows or Linux or anything that will run python.
Has it's own server or can be run behind IIS or apache.
Uses it's own object database with full transaction support.
Xhtml and CSS conformant out of the box.
Templating system is way ahead of the pack allowing for a fairly solid separation of design and logic.
Workflow is pluggable. Out of the box workflow includes creator->reviewer->published document...with retraction and rejection that dumps the content back to the creator.
Workflow includes transitions and states, with the option of running scripts in the transistions to perform various tasks such as email notification..etc...
Flexible. The flexibility I see with Plone is in Archetypes. Archetypes are custom content types that allow you to create a schema and a class...Archetypes automagically creates accessors and mutators along with all the forms needed to edit, view, search on your content types. You can then customize those forms to your liking....but even then you don't have to actually create the full form...you just make calls to your fields and archetypes pops in a widget that manages how your data is to be presented and submitted.
I have created custom content types for expense reports...etc..
Plone definitely fills the requirements you have listed. You just need some python knowledge and andy mckays plone book. (you can find this online or pay for the book).
The site's documentation can be a little cryptic until you see how everything works...but the lists are really helpful and really active.