Thanks for your feedback! You can create sibling tasks using the Enter-key and subtasks with Tab.
Tablet: Depending on what tablet you have the web-version might work but we also plan to build native apps.
You're completely right and in fact all URLs where your login credentials are transfered (login, signup, change password) are guaranteed to be SSL regardless of which plan you're on.
Things like login, signup, change password, ... always use SSL. In fact, currently everything else runs over SSL as well (even if you access the site over http you're redirected to https), it's just that in the higher tiers you are guaranteed that this will never change in the future.
Edit: Maybe I should add that I'm the developer of Subtask :)
It can be used as a mindmapping-tool, yes. However, it's actually more a project management tool than a mindmaping-tool, so some features like change the color of a branch are not available but you get all those PM features instead (like a calendar, comments, file attachments etc).