To me the pillar that is most important would be Safety. Safety for the reason that safety is and has always been the main mission of the FAA. By them increasing what is already the safest air system in the world puts the U.S. even further ahead of the rest of the world. If safety is not the main goal of NextGen, then it would be frivolous to implement it. Safety should always be the top priority for every aviation company if it loses sight of that then it certainly will not succeed in the U.S.
The second pillar is the economic impact, which could arguably be the first pillar. The aviation industry globally impacts the economic outlook for all countries. The better the air traffic system, the better results for the economy. According to the FAA civil aviation contributed $1.3 trillion annually to the economy, and accounted for 5.2% of the GDP for the U.S. in 2009. NextGen is going to cut down on delays and create more efficient routes to take. This will lead to a higher profit margin for any aviation company, as well as contribute more to the economy.
The next pillar of NextGen is flexibility. It is always important that when something is created it is susceptible to change There are almost always unexpected consequences of something new that simply cannot be predicted without real-world testing. NextGen needs to be flexible to adjust for any procedures or operations that end up being hazardous to the system. In addition to that, today the world changes really fast and especially technology. Although NextGen is much more advanced than anything put in place for the NAS thus far, it is highly likely that an even more advanced technology that is more efficient will come into play in the future.
The final pillar, which is important but out of the others is the least important is sustainability. The aviation environment in the U.S. has grown to great lengths over the years and is in a pretty good place today. It currently has the safest airspace system in the world, creates great economic benefits, and is one of the most reliable in the world. Sustaining growth in the aviation world is important and it will grow at times but compared to the other pillars in the NextGen plan is perhaps less important.
User fees are a very sensitive topic for a good reason, no one wants to pay for more than their portion of what they think they should owe. However everyone has different opinions of that as well. Although user fees can be great for implementing the NextGen system it will drive costs up for customer's of the aviation industry. A lot of people also fear that if a user fee is implemented with the airlines it could come down to general aviation in the future since they are use the NAS. If this happens flight training costs will rise and it will be much more difficult for those not in the industry to get their certificates due to the increased cost. I think that the fees should be implemented based on your distance traveled. I believe that this creates a fair system that isn't based on how much fuel is burned or on a flat rate. This system would be better than a flat rate because if there is a flight from DTW to LAX and a flight from DTW to CVG there are huge differences that cannot be done by a flat rate. Additionally for different planes they will burn fuel at different rates which also creates an unfair disadvantage to some aircraft. 
