It isn’t really about TFS 15 RC1 this time, but now the UI is totally out for it so it is very prominent 
there is a very nice feature I usually see being overlooked when talking about the new build: meta-tasks.
Meta-tasks were added a couple of weeks ago to the service, and they are a very easy way of collating a set of tasks you keep repeating in your build definitions.
For example, let’s say you start a VM and then extract files to it all the times:
You can group this as a meta-task so you can reuse this set of tasks elsewhere:
Once a meta-task is created its configuration is retained, so you can just drop the meta-task in the definition and you are ready to go!
Meta-tasks can be managed in the same hub as Build and Release Definitions…
…and they come with their own set of ACLs, so you can restrict access to them if you want.
The very nice side effect is that once you start creating your meta-tasks and make them available to the team, you can start bringing a new Definition up to speed very quickly because you already have some building blocks to use instead of relying on heavy documentation and long configurations with variables and such.