This is a great thread topic, Shamrocker. The Bible specifically states not to be yoked with an unbeliever, which kind of leads us to believe that as long as we are both believers, all is well. But I think it's often wise to go further than that with this yoke business.
A yoke is meant for a TEAM of oxen, and when they aren't going in the same direction of course there are problems (the believer and unbeliever). However, there are also problems if they are moving at different speeds, if one is significantly stronger than the other, if they are pulling against one another, etc.
If one partner is much more spiritually mature than the other, this could be an example of being unequally yoked. If one has convictions that the other does not share. Differences of belief in essential doctrines.
From a non-spiritual standpoint, the same goes... clashing personalities, different values or moral standards, different ways of spending money, even differences of cleanliness can cause unequal yoke partners in their daily lives.
In the end, the most important yoke is that of faith in Christ. Other things can and are overlooked successfully quite often. But if you are yoked with an unbeliever, how can you plow a field with any sort of efficiency? Everything that you base your entire life off of - your moral system, your desires, your self-discipline, identity, motivation... the person who doesn't know Christ will never understand those things, unless God changes their hearts.