I got the itch to play a few games over the past weekend. I played with a few early-domination-oriented leaders (Genghis, Attila, Ashurbanipal) and kept running into the same problem. They had no problem toppling adjacent civ's, but by by the time the 2nd one falls, the entire world freaked out and declares war upon you for all time. Balancing unhappiness (annex, raze, war weariness) , unit maintenance fees, and fighting off 3-4 other civ's at the same time is a real nightmare.
It just seams easier to go domination by choosing a Civ with a unique military unit in the mid-game, turtling up through the early game, establishing technological dominance, and then steam rolling through the world.
The more I play Civ5, the more I am convinced that Civ4 is the superior game. Civ5 just feels way too tedious most of the time.