1. The 1 year free tier
This means you can experience all of the Amazon Web Service for free. Including EC2, RDS, S3, etc.
2. Community and Support
Amazon itself do not provide very good support, but there is a community of developers and companies providing AWS related technology support.
3. All in one place
You can find almost all necessary component of you web stack, though some component is not very good, but All in one means it is well integrated.
4. Scalable and AutoScaling
You can scale up to more machine if there are high traffic at some time, and scale down to less machine to save the hosting cost. And it can be automatically scale up and scale down.
5. Cross Region and multi-AZ
You can build up your web stack cross multiple region, this means your website is alive even one region is die. You site is active on multiple active zone.
RDS of AWS is better then any other database hosting. You can have hot standby at another region, multiple master database at the same time.
And Master-Slave setup is also very easy. Backup is very easy and you even do not to care about this.
S3 is easy to use. You can sync your files to S3 bucket with one command, this means you can write to disk, but read from S3.
8. Route 53
Route 53 provide DNS layer health detect. You can use Route 53 as the HA solution for multiple IDC.