You certainly can use WAAD for creating user accounts. You, also of course, have to force users to use e-mail style logins.
There is however one (BIG in my opinion) disadvantage of WAAD against ThinkTecture's Identity Server: WAAD does not have a user registration / password management / password reset flow.
UPDATE (29.07.2014)
Today WAAD provides Self-service-password-reset as part of Premium Features. However still no self-service-user-registration. Frankly I do not expect to ever see self service user registration, as WAAD is targeting enterprises, and not your specific scenario.
To implement mentioned flow in WAAD, you have to developed your own MVC App from scratch, that uses the Graph API for all mentioned scenarios.
On the other side, you have Identity Server, which has thousands of downloads, which is developed by the Gurus of Claims based authentication and security. Identity server has very rich and easy to use extensible structure. While it also does not provide the user registration and password reset flows out of the box, it is already an MVC 4 application with very rich extensibility points.
Setting up an Identity server for run in Azure is also extremely easy. And setting up Identity Server as Identity provider in Azure ACS is just couple of clicks on the management portal.
You say that WAAD is SLA backed, highly available, etc. But your Identity Server deployment on a Cloud Service will be SLA backed too if use at least 2 instances of a Web Role.
If I have to chose whether to extend Identity Server to support user registration etc, or to create entirely new application from scratch that uses WAAD GRaph API for that feature - I would use Identity Server.