I think emphasizing its energy-efficiency is simply not enough because being energy-efficient is essentially debatable as it is the "price" for keeping the network safe.
But the reasons for being energy-efficient and the consequences of it are worth talking about.
It is energy efficient, because PoS needs only little CPU power (runs minting on my RaPi with < 20% CPU load).
That PoS process secures the network AND helps keeping the decentralization (in difference to the centralization occuring to Bitcoin).
PoS helps preventing the "Tragedy of the commons".
You can (once there are client applications) let Peercoin PoS minting run on your smartphone.
So you can have a secured network without paying a high price for electricity!
The raised security level by having a hybrid process (PoS/PoW), 2 separate processes which secure the network (independently!) is important.
The quite fair Peercoin distribution, mainly provided by the PoW process until now, solves the initial distribution problem for "PoS only" coins.
I think another aspect could be the inflationary model.
It is well-known that PoS generates roughly 1% annual interest.
But it is not as widely known that the PoW coinbase reward decreases rapidly as the PoW hash rate grows (coinbaseReward = 9999/diff^1/4).
It is not widely known that the majority of blocks are PoS blocks.
If you add it up with the tx fee of 0.01 PPC/kB that is destroyed (it is important to pronounce the "per kB" part as it is a minimum fee, which can exceed 0.01 per tx easily!) you find out, that there will be either an equilibrium between created and destroyed Peercoins or at least a very low inflation rate (which is not bad, in difference to hyper inflation!).
The tx fee prevents Peercoin from being used for micro transactions. That keeps the block chain size low (still under 300 MB with > 100,000 blocks and roughly 1.5 years runtime) and the amount of data transfer to keep the block chain in sync low. Another thing that is good for keeping the level of decentralization high.
Inflation of Peercoin compared to Bitcoin: http://cointrader.org/wp-content/uploads/2014/01/ppcbtcinflation.png (taken from http://cointrader.org/peercoin-proof-of-stake-and-bitcoin)
Mentioning Peershares is a great deal. As Peershares has decided to be a Peercoin piggyback the "development centralization" of Peercoin will rapidly decrease. And Peershares development is likely to boost the development speed of Peercoin (as stated by the devs) as well.
It is remarkable that the Peershares development has been funded with coins worth hundreds of thousands of USD!
This way the Peercoin development is funded indirectly.
This is what makes it more and more believable that Peercoin can become a backbone currency.
Peercoin development is indirectly supported by Peershares, because it's needed by Peershares - because it utilzes Peercoins as backbone!