Here's an explanation as to why OBS needed to be rewritten:
https://github.com/jp9000/obs-studio/blob/master/README
Features that have been added to the rewrite that were never in OBS1:
* Mac and Linux support
* Encoded capture device support (HD PVR, Roxio, etc.)
* Filters (color correction, mask, delay, scroll, etc.)
* FFmpeg output
* Mutli-track audio recording
* Native .MOV, .MKV, and .TS output in addition to .MP4 and .FLV
* Visual Theme support
* OpenGL renderer
* Different settings for streaming vs. local recording
* AVerMedia hardware encoder support
* Native BlackMagic device support
Things that still need to be added to the rewrite for parity with OBS1:
* Deinterlacing
* Quicksync and NVENC support
* Image slideshow plugin
* Ability to delay stream
* Ability to select network interface
* FPS indicator on status bar
A few things that we want to add to OBS Multiplatform eventually, in no particular order:
* Ability to preview and edit non-live scenes
* Plugin manager
* Transitions
* Multiple stream outputs
* Adjustable UI
* Better source management tools
* VCE support