The varied type of ships and planes encountered, "torpedoes" and scout planes. It cannot do RSRDC on top of it though, but you do have a re-worked and expanded set of campaign layer files (and a greatly expanded PatrolObjectives set) from TMO v1.7, such that all of the ships added by AOTD_MadMax and others are called and used (which is also why RSRDC can't be used with FotRSU). There are tweaks to sound, environment etc, and it is much "prettier" than TMO. Ralles MpdPack is a very good TMO / RSRDC / OTC mix that is also very pretty to look at. One thing that none of the other mods have is ships that shoot torpedoes not only at you, but at each other. You can see very impressive surface engagements in FotRSU that you'll not see in any other mix for the game. There are also "scout planes" that the BB, CA and CL can "launch" ("spawn" is the correct term). Also, there are enough campaign files that the game probably would not play the same way ten different run-throughs. I know I'm gorfetting something, but hopefully, Rockin Robbins or cdrsubron7 will correct me!
Edit: Whoops, you've posted in the FotRSU thread, but mention FOTRS, and I missed that. This thread is about the "Ultimate Edition" of AOTD_MadMax's original Fall of the Rising Sun mod. Max had release several versions, adding and dropping ships as he made his revisions. We took and combined all of Max's mod versions together, and then added to it. FotRSU also plays differently from FOTRS, due to the edited campaign files.