Goaltending, and their top 4 D have been relatively healthy all year. I think Krug missed 8 games but the other 3 have been healthy.
I don't know how people are saying Boston has had injury woes this season. Bergeron, Krug, Krejci, McAvoy and DeBrusk have missed like 35 man games between them this season and the rest of their good players have been healthy. We already exceed that just from Hyman, Marner and Tavares' missed games, and that does not include the like 125 (and counting) man games lost by Mikheyev, Dermott, Johnsson, Rielly, Ceci and Muzzin. That's 3/4 of our top 4 defensemen and two of our top 9 forwards, and only one of those guys is currently healthy right now.
Toronto, Pittsburgh and Columbus have been really shafted by injuries this year (especially PIT considering who it happened to), but have still managed to be as competitive as a lot of other teams in the East who have had virtually no injuries all year... And Toronto has had substandard goaltending from Andersen and Hutch for most of the year, while Columbus is icing two very unproven goalies who are both doing quite well.