I've been stuck around 1600 as Isis for pretty much the last couple years and I can tell you that nearly every game I lose I lose for one of the following reasons:
1. Inconsistency in the first 6 minutes, especially on fishing or low-food maps like Highland, Mediterranean, Oasis or Ghost Lake. Let's face it, if you screw up your BO or don't scout well enough in Archaic and end up with a 6:30 classical you're probably going to lose.
2. Scouting and raiding. I either don't do it at all or the other guy, especially when he's Poseidon or Odin, does it better than me.
3. Multitasking. The more things you can do at once the better a player you are, a simple example being microing your raiding party while your villies are either building buildings or moving from resource to resource. Another simple example is sending your big army toward his base while sending 6 raiders to an undefended group of villies. The 1700+ players make that look so easy but I just stink at it.
4. Thinking on your feet. This includes simple things like, you see him massing toxotes so you mass slingers with a phant meatshield, or you see a long line of mules running away from all 3 of his TCs so guess what? There's a market back there - go blast it! Or you've scouted and you know where his base and buildings are so you select a few raiders and give them waypoints that will avoid their defenses as much as possible. Of course, if the other guy is *also* thinkin on his feet he'll switch from toxotes to hops and peltasts but you know what I mean - then you have to think again!
I would say that a player who never screws up in the first 6 minutes would be at least 1650, a player who does #1 and also is an effective scouter and raider would probably never see his rating fall below 1700, and then from then on, it's about 10% luck and 90% who does the better combination of 3 and 4? Once you've got 1 and 2 down to second nature, the better you are at 3 and 4 the higher your rating will be.
Sorry if that's a blinding flash of the obvious .