According to github, the changes are:

first new beta:
- fix 'r' in font, it didn't have any extra space in front of it
- fix glow for DEUT, LIGH, and DUST (firework?)
- some kind of fix for the icon on Linux

second new beta:
- fix replace mode delete with tools (won't delete stuff anymore, I accidentally made it do that)
- enable realistic heat, put build info on intro screen

Realistic heat changes the way heat is transferred a little, i'm not exactly sure what it does

Edit: lol, a save with -1 votes was just # 2 on the download page, maybe that should be fixed

@therocketeer (View Post)
Disabled, because it needs more work doing on it.

https://powdertoy.co.uk/Discussions/Thread/View.html?Thread=13192
https://powdertoy.co.uk/Discussions/Thread/View.html?Post=194519

@disturbed666 (View Post)
I disabled NBLE from ionizing at high temperatures because then fusion couldn't happen. I also made hydrogen (and oxygen in the next version if it has oxygen fusion) not burn at higher temperatures either. Just cool them down to make them act normally.

@jacksonmj (View Post)
Why can't it just be made toggleable like other options are? It could be disabled by default. It is like this in my mod.