If there were any exchange matching all 3 of these properties, it would be my first pick:

1) It has decoupled its trading platform from its web platform, so that a DDOS only brings down the public-facing website, and doesn't majorly lag all trading

2) It is smart enough to pause trading when they're unable to process the orders they already have

3) It can prevent bots that place 100 micro trades/minute from swamping their system, if it's something they cannot handle

I don't think MtGox has any of these.  Not sure if any others do.