Want to notify somebody of a new thread you're creating because they are relevant? Tag them!
Need to bring somebody's attention to a thread without going through the trouble of PMing them or whatever else? Mention them! Use @username.
You'll be notified anytime you're tagged or quoted.
Also, hash tags are now available. Say there's something actively being discussed in multiple threads--you can subscribe to that tag to see all instances of it.
For instructions and information, go here. Click on "Using Advanced @User Tagging."