This particular beta will come out mid-April. We will either have newer betas or a release version out by then, but if you like you can also re-install 6.1.1 over the top. None of your registration information will be lost.
We'll have some more feature changes before we go to release with 6.2, but in this beta we've added several performance and quality features.
For older processors, we've added an MMX version of the MPEG quantization method that will improve encoding rate around 22% (when using MPEG quantization).
For HT, SMP, Dual core processors we've added threading optimizations to the built-in resize filters. The gain in encoding rate will depend on your processor configuration and will be most noticeable in the faster encoding modes. I see over 40% improvement in encoding rate on my Pentium D Extreme Edition CPU when encoding in Balanced (default) mode.
We also added an "Enhance threading" option whose benefit really depends upon the particular tools and processes you are using to encode, as well as your encoder settings. In the worst case you'll see no benefit from enabling this option, but in the typical case we see around 16% improvement. In the best case on my PDEE box I see up to 35% gain in encoding rate. For this beta, I'm particularly interested in how this option works in your favorite encoding applications. Let me know if you find any that have problems if this option is enabled.
Finally, we added an "Enhance texture" option that improves texture around keyframes and should reduce stray blocks under high motion (which are particularly visually disturbing at lower bitrates). I personally find this makes a large difference to facial texture around keyframes.
Credit: Doom9 Forums