Integration between the various services. Any individual service is pretty straightforward, but bringing all the pieces together can be a challenge (security, networking, etc.) Billing is also a big deal - your solution can end up being very expensive.
A better way to learn it, IMO, is to come up with some project, and implement it. For example, a typical webpage with a backend db, some storage, DNS, maybe load balancing. Avoid EC2 options, only because that's too easy (it's just a VM).
It won't be incredibly difficult, but it isn't as easy as spinning up a VM on your local machine.
Hope this helps.