Ask ten different people what effective teaching is and you’ll get ten different answers. Here’s a handy definition (let’s call it the willing definition for now) from Grant Campbell, currently Professor of Chemical Engineering at the University of Huddersfield. The original source may possibly be from elsewhere. 
“Effective teaching is inclining people to learn willingly what they would otherwise be disinclined to learn.”
So according to this definition, good teachers make you learn things you wouldn’t normally be interested in, or as campbell puts it Teaching easy interesting stuff is easy. Teaching difficult dull stuff is more of a challenge.
Is this definition useful?
This is an unusual definition, but is teaching easy interesting stuff always easy to do? Probably not. It’s also not always obvious to teachers (or students) how hard or easy things are going to be to learn. Appearances can be deceptive.
Imagine trying to teach somebody something they didn’t want to know or poorly understood. Like the Physicist Akram Khan @ProfAkramKhan, who has been trying to teach the novelist Will Self about Particle Physics, that’s hard (especially with a deliberately difficult student like Self) but the results are entertaining. Most students in higher education are considerably more willing than Self, and more motivated to work their way through the inevitable dull hard stuff that comes with every subject, so IMHO, effective teaching is about both the dull and the exciting.
- Diane Salter (2013) Cases on Quality Teaching Practices in Higher Education ISBN-13: 978-1466636613