Chronata, I like your definition!
Inana, I agree that the wheel isn't Karma. I probably shouldn't have used that word. Karma has a "just deserts" feeing to it, and as you point out, with the wheel, it's not a matter of rewards and punishments--it's just the right time, right place. A matter, as Chronata says, of cycles. I like to think of how enormously popular Frank Sinatra was in his time (girls crushing into see him, screaming, fainting)...then came Elvis (girls crushing in to see him, screaming, fainting)...then came the Beatles (etc, etc). Sinatra didn't lose all his fans, neither did Elvis, but each had their time at the top, then sunk away as another rose.
That's the Wheel. And its message is a reminder. You remind the person on top that their time will not last forever--eventually they will fall and another will rise. Ditto, you remind the person who's low that luck can come to anyone. Think of J. K. Rowling and the incredible success of the Harry Potter books. She was a waitress barely able to feed her kids. Now she's one of the richest women in the world and the single most successful author ever. How did that happen? Do you know how many childrens books tell that very same story about a kid finding out he's a "wizard" and going to magic school? Yet it was Rowling's book that hit gold. Why? Right book at the right time. If she'd written it 10 years earlier, when kids were madly reading Goosebumps, perhaps the books would have remained unnoticed.
The wheel turns, and you never know what or who's going to be the next big thing, the winner rising to the top, supplanting whoever's currently there now. It pays to remember that, to treat people well when they're down--and to be humble when you're up.