I think HP flyers will see the most change. First, I have a strong suspicion that the DM mileage requirements will be the ones that stick. US' program is aligned with the other *A programs. This means that HP plats will now have to fly an extra 25k to qualify for top tier. While I think it would be great if HP kept the 75k threshold, I'm not holding my breath.
If you thought getting an upgrade was tough now, wait until all the US DM members are flying within HP's system. The "roaches" who flew UA out west to get US miles will now be using HP instead. This means more competition even for PHX based flyers.
One thing US DM members will have to get used to is the infinite boringness of FlightFund. US has plenty of promos and offers to help you accumulate miles, while HP has one or two lame-@$$ promos per year.