It's hard to say that I was unequally yoked, because when I WAS yoked, I was definitely not walking with God. And for that reason, God broke that yoke and called me in His direction.
But to speak on defining equal yoking...
It certainly can be as simple as the believer/non-believer situation. But Stilly does present a valid hypothetical. In the case of the 10 and the 4, I am more concerned for the 4 than the 10, which is not what Stillz postulated. If the 10 is truly a 10 spiritually, then their faith is strong, and their priorities are right. But the 4...? The 4 is clearly a novice in the faith, and spiritually weak. While they have much to gain by having a spiritually stronger significant other, their lower experience and new faith is in constant combat with their all-too-familiar carnal nature and worldly past. They haven't conquered their flesh to the extent that the 10 has, and are likely to see the new relationship as a distraction from their faith. Instead of focusing on God, now their focus is on this new Godly man or woman who is with them, and in some respects, that new SO kind of becomes an idol.
When I think of being unequally yoked between two believers, this is what concerns me more than anything. If I'm say, a 6, then I'm hoping that the woman I find is no lower than a 4 (not a big gap, right? I can still lead spiritually, but not become a minister-like figure necessarily because I'm so far ahead) and no higher than an 8 (so that I have someone who might be stronger than me, but not necessarily feel that she is pulling me along). We need to be able to move together as we walk toward God, and someone who's too far in either direction is going to cause one of us to struggle some, I think.
I dunno. I could be wrong. It's happened before. Bound to happen again.