On that we will have to agree to disagree. I think his BIGGEST improvement has been the movement and the backhand and return of serve. That is only my opinion. Return of serve has improved in terms of % of returns in play though the quality of the return still needs improvement IMO.
As for the conditioning I think we also need to remember the following...
a) The improved BH means he no longer expends as much energy running around it. I saw him play live at the USO 3rd row behind the baseline on Court 17 couple years ago when he lost to Gasquet in 5 sets and he was running around the BH quite a bit back then. That takes a LOT of energy. The improved BH means he trusts the BH more so he no longer needs to run around it as much and that automatically makes his game more economical. (You will recall that match was on that Labor Day when there was the debacle at the USO with the rain delays and the Fed match got moved to Armstrong. I had row A Loge tickets for the day session on Ashe on that labor day and people were pissed b/c they moved Fed to Armstrong. Of course Armstrong flooded up pretty quickly with a lot of GA ticket holders and a lot of the Ashe ticket holders myself included were left fuming. )
b) By coming into net more he abbreviates points - this also helps to save energy. He does need to improve in his efficiency up at net IMO but the fact that he is venturing forward does help him conserve energy in his fuel tank b/c coming in abbreviates points.
So I'm going to argue that his conditioning wasn't really that bad before. The weaknesses in his game and the way he played just caused him to burn fuel faster.
The changes under Ljubicic/Piatti - the improved BH and venturing forward more has led to a more economical game which saves fuel. Clearly, when you run around the BH all the time like he did before that expends a LOT of energy. He doesn't do that nearly as much anymore. Just by not running around the BH as much automatically saves energy independent of any conditioning improvement. Has Milos' conditioning improved? Well every young player probably puts in conditioning work so his conditioning probably has improved (conditioning of most young players who put in work does improve) but I really don't think his conditioning was that bad before - his game was just burning fuel a lot faster. So I think the conditioning improvement is not nearly as big as the improvements in the BH, ROS, defense/movement.
So based on what I've written above I will stand by my original assertion that the biggest improvements in his game has been
BH, defense/movement, ROS. Though I do agree with you that thailover's comparison was invalid.
For what it is worth I just took the liberty of tweeting Brad Gilbert & Darren "Killer" Cahill and asked them for their expert opinions on this subject. I wanted to get an expert's view on this topic. These analysts / coaches have probably forgotten more about tennis than I can ever dream of knowing so I value their opinions. At the end of the day I am just a fan sitting at home watching tennis on TV. Brad & Darren are experts.
For anyone interested here is what Brad & Darren think are Milos' biggest improvements....
Brad's response was movement, backhand and net-play..
https://twitter.com/bgtennisnation/status/604128303029620736
Darren's response was movement & return of serve (ROS)
https://twitter.com/darren_cahill/status/604178273090826240