Top 3 thing of interest introduced in Angular 4 RC

if...else syntax in component HTML templates

 <ng-template #fetching>
   <p>Fetching...

 </ng-template>
 <p *ngIf="auth | async; else fetching; let user">
   {{user.username }}
 </p>

Saperate library for Animation

In Angular 2 Animation were in library @angular/core since angular 4 it separately maintained in @angular/animations

Strict Null check in Typescript

 let x = String | null
 let y = Number | undefined
 let z = Array<String> | null | undefined