Ability to define capacity at a master release level to allow capacity planning for teams working across products within a release cycle.
I second that, see my remark on "detailed capacity planning", for the same reason.
However: capacity should also be regarded independent of any release - as this is reality -. You may have resources working on parallel releases. What you really need is to see when an allocation (feature/reqs) done from within whatever release overloads a resource - regardless the release.
All releases that have features outstanding at the moment the resource is overloaded should be marked for review. (So Axosoft's Release Planner, or Microsoft Project's Team Planner)
Definitely got my vote. Though I would define it more generically: add capacity planning on any release level.
I hit this hurdle recently too.
I have a relatively smallish cross functional team that works across several related, but independent, products within a portfolio. Currently i have to track capacity for each product/version in Excel to ensure i don't oversubscribe the team for a sprint.
It would be good if Master Releases could represent sprints, and the estimates for each product/version contained in the Master Release was rolled up into the Capacity for that Master Release.
That way, at the Master Release level, for example, i could see that i had used up 100 out of 150 hours, across 5 projects, so still had spare capacity.