[stigmata]

At the lowest stakes it is "easy", depending on how you define easy. You have to know the value of position, and which hand are statistically playable in which positions according to the action before you & the table conditions. After the flop, you have to know how to protect your hand and how to draw correctly. If you can do these things (a short 200 page book will do) you can play winning limit poker at the lowest stakes. However, the edge is small and the variance is high. You will hardly be able to grind out a living.

Therefore you need to play mid/high stakes to earn a decent wage. But now you have the problem that there is a lot of money at stake. People like money, and you will find that the majority of people at your table have read a few poker books themselves. They also know how to protect their hands, draw to odds, the value of position, and a whole bunch of other tricks: Blind stealing/defense, semi-bluffing, when to run a bluff, how to leverage a table image, etc.

So as you move up, your edge becomes increasinly thin. Whilst table selection is important - you need a donator or two - you don't want to be giving your profits away to the better player. You have to start reaching a higher level of thinking -- what hand does my opponent have? What does he think I have? What does he think I think he has? How can I use this information to make him misplay his hand?

At the lowest stakes, poker is easy to beat in the long run with a small amount of technical knowledge. At the highest stakes, it is a hugely complex blend of maths, psychology and strategy. Like chess, I doubt that most people have the capacity or willpower to master the game at the very highest level.