I am not a big fan of zone of control as it exists now. I think that it may even be unnecessary considering the other rules that Elthos has in place. Part of this feeling is undoubtedly created from my familiarity with D&D 3.5's Attacks of Opportunity rule.
Let's consider the spirit of the rule. To me, a zone of control suggests that an enemy controls "his ground," and is unwilling to let another enemy through.
There are two assumptions here which artificially limit player movement options.
-The enemy wants to prevent movement through a zone of control.
-The enemy is capable of preventing movement through a zone of control.
As ZOC is now, only a skilled (stealthy) opponent can move through ZOCs.
I have the following suggestions, which come from existing Elthos rules.
Allow characters to spend 2 movement points to engage the next visible enemy that moves within reach. This establishes an effective ZOC. If someone wants to move by someone else, they risk getting hit. This establishes the blocking character's intentions and his/her ability will determine the result. It also prevents one character from preventing a lot of people from running by him. Realistically, no matter how skilled you are, if several people want to run by your left and right sides, you can't stop them all.
Other than that, the rear facings, and the attack modifiers that they provide, are more than enough to inspire good tactics; especially with more than one opponent. I also think that should a character try to squeeze through the adjacent squares of an enemy, every step should be accompanied by a pivot toward the enemy, or risk getting attacked in the rear.