For one, when he was on the ice, good things usually happened; at even strength, Shots For exceeded Shots Against by about 10%, and the Goals For exceeded Goals Against by about a third of a goal per 60 minutes of play. He's been effective on the penalty kill, and, much like Greg de Vries on defense, has filled in without a hitch in a variety of situations.
Off the ice, it's been mentioned often that Gelinas in particular has helped mentor Jordin Tootoo, and that may help explain #22's outstanding season, as he's combined his high-octane physical game with discipline (as reflected in all the penalties he's drawing from opponents) and a greater offensive awareness. Tootoo is a much smarter hockey player this year, and Gelinas gets a good deal of credit for that by all accounts.
Assuming Gelinas' surgery and recovery goes well, it will be interesting to see whether Poile resigns the veteran forward. If Steve Sullivan returns and Nashville drafts a forward worthy of making the lineup next fall (thanks to Florida's #1 pick and dismal record, the Preds have a shot at #1 overall), things could get pretty crowded up front.
Either way, it's a shame to see Gelinas' season end so prematurely.
One last note, the Tennessean article linked at the top also reports that Jed Ortmeyer will be determine this week whether he needs season-ending surgery as well for his torn ACL.