Are there any specific requirements for module development and in site architecture for running "heavy" drupal site in several ec2 instances with load balancers?
There isn't anything special within EC2 when compared to other multi server options.
A thing you do need to be aware of is files and how to share/sync your files directory across multiple servers - there are various ways of doing this including CDN, NFS, GlusterFS, cron based rsync. By files I mean the directory specified in /admin/settings/file-system
There was a pre-configured AMI at one point called Project Mercury, but I'm not sure what the state of that is now.