Most of us know good style when we see it. We also know when a sentence seems cumbersome to read. However, though we can easily spot beastly sentences, it's not as easy to say why a sentence - especially one that is grammatically correct - isn't working. We look at the sentence; we see that the commas are in the right places; we find no error to speak of. So why is the sentence so awful? What's gone wrong?
Think about what you were expecting from the sentence. Sure, you wanted information. It would have been nice if the sentence had that nice little "ring" that sentences sometimes have. But most of all (I bet), you were looking for clarity. You want to read without sweating. You feel ticked off when you're working too hard. You wish the writer had worked harder, taking the time to write sentences that are forceful, straightforward, and clear.
As tutors and writing assistants, it will be your job to instruct writers in composing these kinds of sentences. In order to provide this instruction, you're going to have to think long and hard about sentences - what they do, and how they're built.