Sorry I meant mpeg-4 part 2 and mpeg-2 here.

The difference between direct spacial and temporal will be trivial. As explained here http://forum.doom9.org/archive/index.php/t-143904.html by Dark Shikari, an active x264 dev, 2-pass encoding is no more efficient than crf.

"CRF, 1pass, and 2pass all use the same bit distribution algorithm. 2-pass tries to approximate CRF by using the information from the first pass to decide on a constant quality factor. 1-pass tries to approximate CRF by guessing a quality factor over time and varying it to reach the target bitrate."

Here http://forum.doom9.org/showthread.php?t=134545 he says "2pass is not measurably better than CRF, in general."

The idea that multiple passes increases quality is left over from the time of mpeg-4 part 2 and part 4 encoders where this was the case.

As for the sliceless multi-threading used by x264 there should be no significant quality loss unless the number of threads exceeds video_width/mvrange, so it depends on what you mean by a large number of threads and what you consider a reasonable mvrange to be. If you are unhappy with this limitation look at how x264farm works: http://omion.dyndns.org/x264farm/x264farm.html. It splits a video at scene-cuts and allows the scenes to be encoded in parallel as mentioned earlier. And yes it does work with multi-pass encoding.

x264 has supported frame-based parallel encoding for a long time now and it definitely does not result in significant quality loss. This works because the motion vectors are usually limited to 16-24 pixels in length so subsequent frames can start encoding after only a small portion of the current frame has finished.

That would also make perfect sense with crf encoding and there's hardly any reason to use 2-pass encoding over crf encoding unless you are still burning your videos to optical media.