A slightly simpler (!) solution is just to install as you would on any other VPS. I've been offering a free image for a few years now... lately I've concentrated on the new Sydney DC due to it being local - more details at http://www.greengecko.co.nz/magento_on_amazon_ec2 if you're interested in that. Practically zero pain getting started - one click and you're there. Point your browser at the instance for more details. This will make a good starting point - but look in and modify /etc/rc.local if you're going to build upon it.
The important things to realise is that the instances are pretty low powered. Obviously throwing a lot of money at the app does improve this, but for even a moderately small webshop a medium instance is an absolute minimum, just to get multiple cores, and really large is the smallest necessary.
Also, Amazon storage is slow. Because of that, it's even more important than usual to deliver everything you possibly can from memory: tuning databases, memory backed caches, etc are imperative.
Once you get that sorted, it works ok. the requirement to run in a VPC if you want > 1 IP address is really annoying ( especially if you don't realise this when you start out! ), and really the only gotcha you will come across.
It's simple to expand the platform 'on the fly' - eventually the only bottleneck becomes the amount of processing power available to PHP ( inefficient code aside! ), and running multiple 'engines' in parallel is probably the simplest option - bringing extras online when necessary.