First post ever, and to the people who think Yama is "too powerful"?
1. Aizen had everyone in SS under hypnosis so cpt. commander using his bankai would have been folly when he's not even sure that he's fighting aizen. Also, when your the top dog, and have been alive and fighting for at least 2000 years, you don't just throw caution into the wind and potentially turn all your subordinates and, humans into ash unless there is Absolutely no other choice... and guess what??? there was another choice, and his name was Ichigo AKA the hero of the story( who by the way was not under Aizen's illusion) which is why he raced back from HM to save them.
2. As far as we know, Yama hasn't used his BK in over 1000 years. Keep in mind that all that has transpired in bleach has happened in the last year or two. That span of time is the blink of an eye to Yama. When your that old, you do nothing hastily, especially when your unleashing the most dangerous and powerful technique in all of SS,HM, or the RW.
3. Even as powerful as Yama is now, he is still No Where Near as powerful as Mugetsu Ichigo. The strongest 2 dimensional being ever is still a bug compared to a 4 dimensional being. And even though Ichigo had only been a shinigami for a short while, he still had a vast amount of reiatsu(which is what really matters) when he fought Kenpachi, so that argument is irrelevant. Yama's power is totally within the bounds of possibility. What did you think would happen when Yama BK'd? that it would be like lighting a bottle rocket on the 4th of July? Crazy
5. Obviously, if Yama is serious enough to now go BK then he knows something we don't, like Bach is dangerous enough to require him to go BK in order to kill him, or that he just hates him that much. Either way his power level is totally believable, and in truth I would expect nothing less from the strongest shinigami in 2000 years, and consider yourself lucky that Ichigo is incapacitated at the moment, or we still might not have gotten to see Yama go nuts... even though he's as good as dead already.