If PG had a flaw, it is that the units had a very "scissors/paper/rock" sort of balance to them. Once one cracked the formula, one could counter almost any situation with the right combination of units. And once one upgraded the core units to the "super" versions (like Tiger Tanks) then they were almost overbalancing.
On the plus side, the game had a great feel to it, immersing you with flavor and making you work hard to get your core units to gain experience so you could upgrade them. And if you lost one of those cores, it was a real bad day!
One thing I must say that I liked about the scissors/paper/rock system (despite my earlier comment about how the system was a bit gamey) was that it did make players think about the relationship between different weapons systems and units. So you learned something about the synergy between air power, air superiority, AA defenses, blitzkrieg tactics, AT units, engineers, recon, etc and how to use them together. No stack of doom for sure, but more like learning to build a super team that could be used with tool-like precision to crack open almost any defense. The tricky part was to not "use up" your best units doing that.
So I am looking forward to how Civ V will take these ideas and run with them...