Recently I started a prototype involving Big Data processing on EC2. I started with a "guesstimated" size for EBS volume. As in any POC / guesstimates, I was wrong and very soon the size grew and I needed to increase the space. But I realized that we cannot dynamically increase size of EBS volume of a running instance. That seems to be a problem to me. In my opinion, in today's world of visualization and pay as you use model, the vertical and linear scalability should be without any down time.
Anyway, since I was doing this as a POC, I was able to afford a small down time. BTW the process for increase the volume size is not that difficult. Here are the instructions from another blogger: http://www.e-zest.net/blog/simple-steps-to-change-size-of-ebs-volume-in-ec2-of-aws-using-aws-console/