The badboy!Klaine trend. Blaine or Kurt is Troubled, but Cute, and the other is usually very close to their canon persona. Sometimes, especially when Blaine is the bad boy, Dalton is some sort of reform school.
A good number of Kurt/Blaine fic reference or incorporate "Baby It's Cold Outside", even before the actual duet was released.
An interesting trend is several stories where Rory and Sugar sleep together, and she becomes pregnant shortly before he returns to Ireland.
Many future fics involve characters performing together on Broadway. This is particularly common with Rachel/Jesse fics.
Sugar, Rory, and Harmony being the children of Brittana, Klaine, and Finchel or Faberry (respectively) travelling back in time to visit their parents. Occasionally they also include Sebastian as the son of Kurtofsky.
Many Sam/Rory fics take place either during Christmas (when the ship was first introduced) or Valentine's day (in the Christmas episode Sam promised to be Rory's sponsor and help him get a girlfriend for valentine's day).
Similar to the Broadway example above is the tendency to write future fics or AU fics where one or several characters are famous actors or singers. This is especially common in Kurt/Blaine fics, and quite often only one of them is famous and the other is a fan, and they somehow meet and fall in love. These types of fics also usually include parts where the famous character's fans freak out on Tumblr in reaction to various events in the fic.
After Cory Monteith passed away, many fics explored what would have happened if Finn had lived on rather than dying with the actor.