Legendary status is akin to P4P status to me: it's completely subjective.
That being said, there are any number of criteria, in any combination, that can qualify a fighter for legendary status: wins, KO's, subs, marketability, opponents, length of career, personality, etc, etc.
Lyoto, in my opinion, isn't there yet. But he's well on his way. He won a championship; he brought new fans to MMA as he emerged to become the most popular Karate-based MMA fighter; he had a lo-o-ng win streak; he's fought quality guys.
If he can re-invent himself, perhaps at MW, put a win streak together there, and win a belt, he could qualify as legendary in my mind.