I'll give it a shot, but first, a list of disclaimers: Yudkowsky gets a ton of unjustified internet hate and scorn, and I disagree with a lot of it. I read a number of the sequences, and quite enjoyed them. I also think his reaction to Roko's Basilisk was pretty reasonable: someone on your form comes up with a way to basically guarantee eternal torture for anyone who reads it, and then posts it, thus guaranteeing eternal torture for your forum readers? Who cares that the idea won't actually guarantee any such thing, Roko _thought_ that it might; I would be pissed as hell.
Anyway, my point is, I'm only offering a gentle and hopefully reasoned disagreement with someone I regard highly. I'm not jumping on the "fuck that guy" train. Moving on.
There was a post that boiled down to the question "Would you rather 3 ||| 3 people (where | is an ascii stand-in for Knuth's up-arrow notation) get a mote of dust in their eye, or one person be horrifically tortured for 50 years?" and his conclusion was basically "you can use math to assign some incredibly small epsilon of suffering to getting a mote of dust in your eye, but eventually, if you sum enough people, it's more suffering overall than one poor person getting horrifically tortured". My problem with his post was that I think that any morality scheme that results in the person getting tortured is fundamentally flawed, and I don't care how much math you throw at me to try and "prove" that it's better.
I don't think I've ever heard of an attempt to rigorously derive morality that I agree with. Morality is too contextual, too messy, for us to perfectly capture in those sorts of models. It's especially bad when we try and model morality mathematically, and then take as gospel the result of that model, rather than say "oh, um, that's not a great result, the model must be wrong".