"On diety, it is a must to attack early."
It is not! One of the main problems is that people tend to take one good look at thier score very early on, decide it's hopeless and then quit. First off, it's much more efficent to only produce settlers once you already have a granary and have about 5-6 pop. You can expand much faster that way. Second, because of the god awful corruption, the only real reason to control territory beyond about 20 squares from your home is for resources, and if you try for a ssc you can trade techs for them, if you try for a culture civ you can make the enemy cities revolt.
Deity level players will expand faster then you, but it's of limited benefit.
"I can overlook the Roman unit because you get the 3 defence so early, but the other two are not as forgiving. You can emulate a bowman army by building archers with a few spearmen."
Not really. For example, lets say you have 12 cities. You need a defense, so 2 bowmen a city. You need an offense, that's another 10 bowmen. This is 34 Bowmen, nearly Equivalent to 34 Archers and 34 Spearmen. One half the cost. Granted, it isn't quite 34 spear and archersmen, but you wouldn't waste spearmen on offense or archers on defense would you? The problem is that when you lose 1 bowman, you lose the equivalent of one archer and one spearman, but they get to see more action, and get thus get upgraded more.
"But is it not more practical to built two pikemen and one samurai than two samurai? My point is that civs that give an extra point to their UUs strength have the advantage. Your strongest defender gets the hits in a group, so I always send some defensive units when attacking. "
Having a 4/4/2 defender is awesome. While a spearman has roughly a 0.21 chance of not losing vs a knight, and barely any chance at all of actually killing the knight, a samurai has almost a 0.76 chance of KILLING the knight, when fortified, and a 0.84 chance when foritfied in a city. Samurai cost 50 shields, 2 spearmen cost 40 shields, and the samurai will see more action, stay alive much longer, and can instanly go on attack when there is no more danger to the city.
"This is true. In the long run, certain traits win out, although with pop-rushing, I find that a 60-shield temple is easier to build than an 80-shield library, so I prefer a scientific civ. But I get killed by Anarchy with all the gov't switching I do during wars. "
There are, in particular, a couple incredible unique units. One is the Mounted Warrior, the other the immortal, and finally, the panzer. Each of them, during thier period, can be used to such effect that I almost consider it imbalanced.