There can indeed be a runaway problem if one person owns a much larger kingdom and his leaders are surrounded by lots of temples. In my experience, among good players, such a stage is not usually reached, because everyone realizes the consequences. Most of my 3-player and 4-player games vs good players come down to the wire, with the outcome still in doubt until the last round or two. However, runaway is quite possible with new players and in 2-player games, where there is no one else to gang up on the leading player.

One way the runaway stage can be reached is if everyone builds his own kingdom, and then two of them meet and one side wins big. This is especially possible for new players who don't know how to build structurally sound kingdoms, so that one guy can launch a tactical strike and after winning, a large chunk of the loser's kingdom is transferred to the winner's side. As a corollary, if there is "mixing" of leaders, i.e., a kingdom containing leaders from different players, then in my opinion runaway is less likely. Also, if everyone builds structurally sound kingdoms which don't break apart as easily, then runaway is less likely. Over-reaching for treasures is probably the most common cause for beginners to build fragile kingdoms. Treasures must be grabbed greedily but carefully.

New players also tend to not use disasters effectively. This is a very important way to attack the leading player. You can use disasters to cut up a connected kingdom, to destroy temples next to several of his leaders, etc. Since every player has only 2 disasters, the leading player enjoys no advantage in this aspect. It is hard for anyone to survive concerted bombings from 3 or 4 disasters, let alone 6. 4-player E&T games can be quite short -- 10 or 15 rounds or so. If you lose big, there is no time to catch up, which may contribute to the perception of runaway leader. For this reason, I prefer 3-player games to 4-player games.

I remember when I first started playing E&T, all our games look similar -- each one has his own kingdom, then there is a big war, etc. I thought the game was very narrow. Then to my surprise I heard that other groups also feel all their games look similar, but their games look very different from ours!! Early monuments, or mixed kingdoms, etc. The game space is actually very wide. To see more variety in action, you might want to play online at boardgamegeek, where you can meet players of all experience levels.