Generally, the modifications fell into four different categories: hardware, graphics, realism, and multiplayer connectivity.
Hardware meant making things like NatualPoint TrackIR (6DOF!) work and adding support for multiple controllers (and more recently, porting data out to a separate monitor!). Graphics overhauls were massive improvements and additions to terrain textures (not just making them either, but using new techniques like mip-mapping and texture blending) and vehicle (air and land) skins. The look and rendering of water was improved. Photo-realistic scenery was imported and flight over eye-popping terrain replaced the dull repetition of the original EECH.
New explosion graphics (modified to the extent they could be) replaced the “orange dome” explosions and persistent smoke effects were added.
Outstanding work was done on the Apache that was brought in from EEAH to get the cockpits more functional and the MFDs back to their original crisp and clear resolution (a personal favorite modification I was happy to see done!).