I am going to investigate the meaning of "Improved random seed generator to select worlds with a better spread of biomes".
I think it means that when you leave the seed blank, the game picks one out of... say five random seeds, that has more types of biomes than all the other 4.
I have not confirmed this yet.
Go for it... definitely no additional options available to the player. I suspect that, with random generation, there is now just a minimum number of different biomes that will generate... making worlds with a larger variety of different biomes just more likely (i.e. just improving the odds a bit). I don't know how you're going to confirm that the program is analyzing "better" as being the one of five with more biomes. I do think, basically, they interpreted "better" as "more different ones" based on the majority of people looking for "all biome" seeds (as opposed to those of use looking for larger biome, single-biome seeds) and so inserted a minimum such that the program will not ever randomly generate a seed with less than X different biomes.
ETA: I've generated 6 random seeds so far: 1) all have had all wood types in biomes (so this might be a stipulation) 2) all have had mesas, no bryce so far 3) all have had mushroom islands (definite increase in the odds of generating these) 4) only 1 has had a village (it had 2, one virtually under water at the edge of the world) - may generate less frequently than before 5) none have had ocean monuments (less water overall = less chance of ocean monuments generating) 6) none have had ice spikes (my system still does not seem to want to generate these, although I've also not had a freeze up yet) 7) none have had a flower forest and only 1 has had a sunflower plains. No overall change in the size of biomes that I can see. No significantly high peaks yet either. Land now regularly generates to world's edge.
ETA; World #7 had no mesa. World #8 generated without a mesa and without a jungle. Still no ice spikes. Other than a preponderance of mushroom islands (still getting 1 in every world) and land generating right to the world's edge, I really don't see a significant difference. Not planning to generate any more worlds today. Let me know what you determine.
I have spawned snowy rabbits in a cold(not snowy) taiga biome with a spawn egg. This does not match the spawning rules on the minecraft wiki for 1.9(which they are based on). It might be a bug, so I am reporting it.