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.
It it as simple as taking each individual service listed in the AWS console individually and learning exactly how they work, or is there something more in-depth that matters?