Why is Mongo stuck in STARTUP2?
The answer of eoinbrazil is partly incorrect. A new Node can be in STARTUP2 for a long time. The link the posted says: 


  Each member of a replica set enters the STARTUP2 state as soon as mongod finishes loading that member’s configuration, at which time it becomes an active member of the replica set. The member then decides whether or not to undertake an initial sync. If a member begins an initial sync, the member remains in STARTUP2 until all data is copied and all indexes are built. Afterwards, the member transitions to RECOVERING.


I'm administering a 700 GB collection and, when I add a new node the STARTUP2 state remains well over 24 hours. But you can still see if there is something happening, by watching if the database grows. You can see the size of the database on the new node with

show databases


or you can also observe the data directory, to see if it is still growing. (on linux with the commands ls, df, du, iotop, etc ....)