I also have to re-train my brain to use left and right instead of up and down to target party members (my main is THM).
I do wish that they had kept the up and down for party member selection as the party listing is displayed vertically on screen so it is only natural to expect the cursor to run up and down the names in the same order/direction.
Selecting dead party members seems to be broken for me now as pressing left and right with "All", "Party" and "Friendly" doesn't seem to pick them up. Using F1, F2 etc on the keyboard works for the most part (in parties with just a few people), although in a large leve party the other day I still could not find the F key number associated to a dead member... maybe it was just me.
Also, even with "Enemy" targeting selected, people still automatically target the healer when buffs/cures are cast on them before engaging a mob.
The left and right party member selection is only for those in line of sight (where your camera is pointing) as someone mentioned before, so some of the original targeting functionality has been messed up in my opinion*shrug*
Probably not so bad for melee/dd, but as a healer I definately notice it.
I think the change is a step in the right direction but I would personally prefer:
a) to have the target selection function not assigned to left and right or up and down
b) the up and down put back to party/self selection
c) possibly the choice to turn the function on and off (rather than a default "here it is in your face!").