One of my biggest concerns about using Amazon S3 for my storage space was the uncertainty of my monthly bill. With the potential for other sites to 'hotlink' to your content and no ability to cap your charges, the bill could escalate to much more than you intended.
Luckily, Amazon S3 is cheap. For months when I was simply using it for backup storage I wasn't getting billed more than $0.03 per month. There are steps you can take to prevent problems like hotlinking (like signed urls), but it's also nice to be able to keep tabs on your bill when it is reaching certain thresholds. Queue billing alerts...
You can setup as many billing alerts as you like for different thresholds, so if your requests start getting high you can take a look at the logs and review if you need to take any futher action. Billing alerts can be setup in the amazon s3 console with the following steps:
Whenever your bill hits the threshold in future, you'll get a notification!