Most add-ons will be limited to the particular game they're built for (the Final Doom mods are an "exception"). However, you'll be glad to hear that the developers are lifting engine limitations to allow for MegaWADs, and they're even introducing real-time MIDI playback to preserve the custom music in some mods.

There are some notable practical improvements beyond this. Both Doom titles now run at 60 frames per second on all platforms instead of the earlier 35FPS. There's an aspect ratio option to match the 4:3 output of the original game. You can quick save (and quick load) if you're worried you won't survive an encounter. Level select lets you skip to a favorite map. And if you're playing on a device with a gamepad, quick weapon select and a weapon carousel will help you quickly switch to the gun you need.

In many ways, this upgrade is as much about preserving the spirit of Doom as it is enticing modern gamers. The initial releases are widely credited with nurturing the very concept of game add-ons -- it'd be difficult to convey this to a new generation of players if they were forced to play the built-in levels and nothing else.