My worst gripe with TS12 is copy and paste in the surveyor route editor screws things up, and a few other bugs have been reported by others.
For pure running tho, testing with the same routes, same trains, same locations, same camera angles, and same display settings there's no difference at all between TS2010 and TS12 in either appearance or performance on my system.
That's where a lot of the conflicting opinions come from, one guy tries a simple route in TS2010 then tries one of the heavily detailed routes in TS12 and says the framerates in TS12 suck - comparing apples and oranges, the same route imported into TS2010 would give the same FPS - if all the display options and detail sliders are set exactly the same in both games, the most common thing that testers miss.
TS12 has a volume control, a slider to reduce tree detail, and doppler effects, along with new routes and trainsets. I never used compatibility mode in TS2010 anyway, so I really don't miss it, I'm just waiting for all the patches to be patched before I move route development from TS2010 to TS12. I am periodically saving my WIP route to a CDP and installing in TS12 to check, some of the dependencies have those yellow warnings about no thumbnail and obsolete build number, but it runs the same in TS12 as it does in TS2010.