This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
I suspect that using maps, lists and beans with EL expressions is one that many people do not quite understand. In fact it's not that complex and can be learnt quite quickly. Hmmmm some kind of cheat sheet would be useful.
It is easy to get the details of the JSTL tags wrong.
I agree that design patterns can be a tricky one. Most people get MVC easily enough but the other can be a little tricky.