Justified Expectation of Pleasant Surprises
The main fun and rewards structure in Diablo 2 (which I assume you're talking about - that or a clone) is the equipment, which you can't predict before you get, and which drops nearly constantly (with good equipment a bit rarer, but you're always assaulted by limited inventory and having to prioritise whether you need a piece for your own use, to sell, or whether you'll just leave it). The skill trees are there for the planner and optimiser - you can't really make bad decisions by choosing whatever sounds good, since your options at any given level are limited, but you can look ahead, sure - that prevents you from picking up "speedbump" powers that don't lead anywhere interesting.
Besides, just the text of the skill is hardly the reward for getting it. There are lots of properties of the skills you only see once you use the skill or even level it up some.
I think in general action and exploration games have a better "fun" structure though. Check out the talk here (begins about 10 minutes in) about the fun structure of learning used in most games and how to apply that to non-game applications.