It's an interesting question and one I've waffled on quite a bit over the years. At times I feel like an ace is someone who has been one of the top pitchers in the league consistently over a period of time, not just over the course of one season. At others, I think it's a pitcher who has had a truly dominant season and that it's okay to say "He's been an ace for this season."
Ultimately, for me, it requires excellence beyond being the best pitcher on whatever pitching staff you are on. Of course, limiting the term "ace" to the Pedro's and Koufax's of the world would render it redundant and useless as we already talk about those guys as being the greatest ever at their craft. It is, however, difficult to parse the idea of an "ace" out from the idea of a hall of fame pitcher since there is going to be so much cross over between the two groups. Being one of the best pitchers in the world, even for a season, will usually mean a talent level high enough that they should be sniffing the Hall of Fame when they hang up their cleats so long as they were healthy enough to log the innings necessary.
Right now, if I had to name every pitcher I consider an ace, the list would be Kershaw, Price, Verlander, Hernandez, Lee, Scherzer, Hamels, and Sanchez. Doug Fister is just outside of that list for me, and both Sale and Darvish are banging on that door waiting to be let in. Jose Fernandez looks like he has the talent necessary to get there if he can stay healthy and maintain consistency. Matt Harvey will probably get there if he recovers well from Tommy John surgery. Sabathia pitched his way out of that list in 2013 and Verlander's season could have been a small step toward declining into something less than an "ace" but might have just been a brief blip where he was merely really fucking good instead of otherworldly. 2014 should tell us a lot there.
This is obviously all very subjective, but I would have a hard time swallowing a definition of "ace" that included more than a relatively small group of pitchers.
