John McFarland is a very good bantam player. However, he is not close to being exceptional in the way that the Hockey Canada guidelines require: a player akin to Crosby, Lindros, Lemieux, Gretzky and Spezza (those are the names used in the guidelines).
When Tavares was granted exceptional status, it was after a season in which he was the top scorer on the best team one age group older. He also had excellent stats in a handful of Jr. A games.
McFarland has played with his own age group for arguably the best bantam team in Ontario in the past 5-10 years. They are something like 70-0-2 on the season, and they likely could have been undefeated without McFarland (they easily beat Honeybaked early in the season without McFarland and Chase Balisy in the lineup). He has played a handful of games one age group up, many of which I have seen. In some of those games (recent Rebels game) he was outstanding, in others (recent Marlies game) he was invisible and clearly struggling with the size and strength of the other players.
In fact, I'm not convinced that McFarland will be a great OHL player, let alone worthy of exceptional draft status. I'm really not sure that his game will translate at the next level. He dominates his own age group right now because he is one of the bigger, stronger kids (about 6'0", 175 lbs), he has above-average speed and he has a tremendously quick shot. However, 90% of the time he tries (and is able) to beat the defenseman on the outside - that won't work in the OHL nearly as often. He is able to beat goalies in his own age group with shots that are routine for goalies in the OHL. Finally, he is quite the bully and stickwork artist, which allows him to intimidate opponents in his own age group. Obviously, that tactic will be much less effective in the OHL, especially given that it doesn't look like he is going to grow that much taller.
Bottom line - there are at least 5 1991's I would draft ahead of McFarland without even thinking about it, so I'm not sure why it's necessary or warranted, from the OHL's perspective, to grant him exceptional status. The original purpose of this process was not to simply let the best player in an age group be drafted a year early.