Session Play in Lord of the Rings Online is a feature that allows you to take the form of a different character, and experience Middle Earth from a new perspective. From the official website, "You could play as a Ranger, a Troll, or... a chicken." Well, it looks as though your only option for now may be of the feathered farmyard variety, according to this post on Turbine's official LotRO forums.
The announcement from Turbine Community Specialist Saffron tells us little other than that there have been unspecified issues with Trolls and Rangers in Session Play, and that they will not be available until further notice. These two character types were formerly playable in the Ettenmoors Monster Play (PvP) zone, with Rangers for the Free Peoples and Trolls for the Monsters. A quick visit to the Monster Play forums shows that Trolls and Rangers have already been a topic of hot debate, with many feeling they were overpowered. Perhaps they will be tweaked along with other PvMP updates due with Book 12 -- we will have to wait and see what changes have been made when they are made available for play once more.