I love to rail against the use of too-long sentences and too-long paragraphs. One of the easiest ways to improve any piece of writing is simply to refactor it into a greater number of (shorter) sentences and paragraphs. Short sentences are easier to parse. They're easier on the writer as well as the reader. What's not to like about them?
Long sentences are an invitation to semantic and linguistic disaster. The more verbiage you ask the reader to "push onto the stack" (in programmer parlance), the more likely it is the reader will forget where you're going before you get there. You're asking the reader to do a lot of work when you present him or her with a too-long sentence.
Want proof? I recently came across the ultimate example of a too-long sentence. Coincidentally, it also makes for a too-long paragraph.
This longest-sentence-ever is from Vol. 4 of the longest novel ever (at 1.2 million words), Marcel Proust's Remembrance of Things Past (À la Recherche du temps perdu), said by some to be one of the top novels of all time. I present the sentence here as an example of how not to write (unless you're a dilettante writing for a 1913 French audience and you don't care if anyone understands you).