Air vehicles in PS2 are completely broken. They are extremely powerful and made even more deadly by the fact that there are no strongly effective ways to destroy them from the ground. They are effective against infantry and armour. Heavy tanks can be virtually instagibbed by a well aimed rear rocket attack from a fighter.
There are weak counters to air: lock-on AA rockets and flak cannons (the MAX's burster). But lock-on rockets are prohibitively expensive. They cost 1000 certs. In game terms, you tend to gain certs at a rate of about 30-40 an hour [edit: actually, I've just done the arithmetic on my PS Universe stats, and it's exactly 30 an hour]. That's (at least) 25 hours of play to buy *one* item, which is comparatively pretty weak!
The MAX has a single burster by default. It's very effective at getting a fighter down to about 50% hit points before it decides it's time to tootle off and repair. There is absolutely no chance of taking down a competent pilot who engages with most of his vehicle's hit points intact. A second burster will have a good chance of killing a pilot who makes a misjudgement, but again, it's prohibitively expensive.
Of course, you *can* unlock these weapons by grinding. But it doesn't make sense to invest your first 50 hours in the game in only two upgrades for your character, especially at the cost of smaller, varied upgrades to your damage resistance and general killing power. In reality, unless you are willing to put in actual cash into the game, it will be a long time before you are effectively equipped to repel air units.
Ironically, air units by default aren't *overly* dangerous against the ground. It's just once they have their own respective (prohibitively expensive) upgrades that they become ridiculous.
You can argue that this isn't technically pay-to-win because all these things are accessible without putting money into the game, but you are rather missing the point. Whilst it is important in any pay-to-win game that many people do not realise it's pay-to-win, PS2 is rather blatant about it and I doubt everyone is quite so blind.
The free to play model is fundamentally hostile to the player because it greatly overvalues the game's content. I can buy a recent AAA title for £30 and get the whole game. If I were to pay for all of PS2's content, I'd be looking at an order of magnitude greater than that. Plus, it's a strange business model. Intuition suggests that there would be a negative correlation between the people who don't balk at the idea of spending £5 on a virtual item and the people old enough to earn money.
Game devs: Learn from TF2, please.
It'll be interesting to see over the next six months how this compares to Tribes Ascend. One of Ascend's big problems in player retention was the amount of grind to unlock items; a lot of players cottoned on fairly quickly that they were being killed by weapons inaccessible to them, approximated the time investment required to unlock them for themselves, and then thought "forget it". (As someone who put in 400+ hours and was frustrated by other things, it's easy to forget that this was a serious issue for new players, but there was an insightful Reddit topic in /r/gaming where a lot of people chimed in saying they checked it out, but quit soon after because of this).
PS2 seems a lot more grindy than Ascend ever was. What's worrying is that the cert prices were cut during beta as people complained they were ridiculous. What's more worrying is that SoE were quite resistant to doing this at first, which, to me, shows that they don't really get their market. This is exacerbated by the fact that earning certs is something that happens directly from certain actions in the game; actions which do not correspond well to doing something useful for your team. At least Ascend had a layer of obfuscation in its XP distribution so farming XP at the expense of everything else was never really an option.
For what it's worth, Amerish on Mallory is a ghost town already and Esamir's not holding up too well either, indicating that player retention has been poor. PS2 relies upon players to a far greater extent than most games; the players *are* the content. Alienating the free to play crowd is incredibly short-sighted.