What is the derech halimud in Mir Yeshiva (Jerusalem)? As far as I know it is not the Brisker derech. What commentaries or chiddushim are learned there?
As per this article, the Mir is composed of a multitude of study groups, called Chaburot.
As no two individuals are the same, each Maggid Shiur has his own style of learning and teaching. However, according to this article, the Mir has a unique central Derech Halimud:
"Despite the multitude of teachers and student in the Mir, there is a common Derech HaLimid, know as Shitas Mir."
"One of the [main] characteristics of this Derech, is a careful analysis of Rashi's wording and deriving a scholarly framework based on the nuances in Rashi's wording."
It's fair to assume that the transcriptions of Reb Nochum's shiurim, published by his students - Chidushei Reb Nochum, Shiurei Reb Nochem and Chazon Nochum - are popular Seforim.
A description of these publications, gives us some more insight into Shitas Mir:
"Reb Nochum followed the Shitas Brisk; each Gemara is explained based on [a careful analysis of] its wording and that of the Rishonim, without adding one's own explanations.
Also see here for a long article in Hebrew describing each Chaborah - where it's located and what it learns.
Interestingly enough, the late Rosh Yeshiva had his own Derech Halimud "matching up the opinions of Rishonim from various places". This Derech Halimud did not become popular in the Mir. (ibid)