^Good question, because it is so easy to complain and point out flaws when you have nothing better to suggest...
I think somehow Tolkien's power of writing lies in the imagery, the landscapes he is able to create in our minds, the description of people etc. He is a master of the journalistic trick "Show, don't tell". Show and describe things and people though their actions, not just saying "He was agreat wizard".
Show me how he was great!
I think perhaps in Saruman's actions or maybe in a "battle" on wills between Saruman and Gandalf we could truly see his powers.
Maybe I've seen to much Deathly Hallows recently but couldn't Tolkien have described how the two great wizards came out to behold each other, how the air tinkled all aorund Isengard, as if all the particles of the air swirled and gathered around the two. Or if the leaves and grass seemed to rustle about their feet as if a massive wind was stirring. Or something like when Tolkien describes Gandalf's battle with the balrog where they fall to the darkest, deepest pits and fly to the highest mountain, and when he laid naked and saw sun and moon pass over him and then he felt life in him again. I would have loved the picture of two powerful wizards battling on wills.. kind of like how they show it in the Chinese movie, Hero, I think it is. Where two great warriors duel in their minds in honour of one of their companion's death. You see them fighting on a lake in the forest as a representation of their mind duel.
So yeah.. I'm rambling.. but these are my thoughts on it... It's hard to explain.