Can someone give a quick back-of-the-envelope rationale for change number 2 (or point me to the discussion)? Does "the peg" mean the median price feed? So now we should expect a discontinuous market price: if we're right at the peg, as soon as someone places a bid a tiny bit above the peg, some short 10% below the peg matches him? Sounds like it could get very confusing for traders.
Confusing? Everyone gets exactly what they asked for. If you short Q quantity at P price, then you get a short position for Q units with 2*Q*P collateral if the order is totally filled. If you enter a Bid for Q quantity at P' price, then you spend exactly Q*P' and get exactly Q units if the order is totally filled.
If P << P' (that is, top of Ask/Short book is much less than top of Bid book), then the system takes the difference.
As for the rationale -- basically there is so much demand to short the dollar, above and beyond the peg. So much that they are willing to pay a premium, in the form of setting a short price below the peg and hoping BitUSD will sink even lower. In the older version, the short-heavy Ask book was moving the price far away from the peg by pushing all the way down the Bid book which was much thinner (I think the sorts of people who are actively trading BTSX right now think BTSX is going to the moon and short dollars, trying to add leverage to their BTSX holdings). In the new version 0.4.11, shorts are allowed to compete with each other for a limited Bid pool based on the premiums they are willing to pay, but the premium now goes to the network in the form of fees, instead of to BitUSD buyers in the form of really cheap BitUSD.