all i see with this new system are a lot of problems.
people are spamming light vehicles, snipers, officers, etc.
Funny thing is I have now been accused of light vehicle spam after games and I do bring about twice as much light vehicles as before simply because i used up my armor and infantry availabililty so fast all i have left to choose is light vehicles and support (which you cant bring without munitions)
On the other hand I have actually learned the value of light values and have won several games because I had the light vehicles. So all in all I like the new system.
As for what was wrong wtih the old system, well at a certain point it became more prone to exploits. Like right before the beta ended, the latest exploit was pioneer/goliath spam which was preceeded by Puma spam. I mean the devs would reduce the cost of an item by 10 munitons or lower the pop cap by 3 and the next morning some players had come up with a way to exploit it.
Now with the availability system these type of units are contained in a category of similiar type or units so instead of simply not brining a Marder so you can bring 4 Puma for the same pop cap, now you would have to give up 4 IHT or ATHT.