It's not about medals all the time, Robbie Fowler is undoubtedly a Liverpool legend, but as far as I know, he never won anything of note with them.
For me, it's about how much you can see the player and the club as one, and who have performed at a (relative) high level over time or who have inspired greatness in the squad.
The difference between Cantona and Ronaldo, and what makes Cantona a legend for me, is that in his short time at the club, he practically molded Scholes, Giggs and a few others into the on-pitch characters they became. (or at least I've read that his influence on the squad was immense and helped them never to settle with a win).
The same can be said for Keane, even if he in recent years has done his best to sound like a right cnut, he clearly was a leader who you could define as a Manchester United player.
Now, you might say that George Best, as a legend, resembles the same as Ronaldo, but for many of us the difference is how we feel like a stepping stone for Ronaldo, wheras Best was our golden boy for 10-11 years. He built up and spent his career at Manchester United. Ronaldo came, was a brilliant player who won a lot with a good team, and then left because his heart was set on another club.
His individual greatness might make him a legend in some fans eyes, and that's a perfectly fine way to look at it.
But that doesn't mean that those of us who don't find him to be a Manchester United legend because of those reasons are buffoons.
Charlton, Schmeichel, Cantona, Keane Edwards, G. Neville, Best, Robson, Giggs, Scholes, (even) Beckham and more, when i hear those names I picture them (those i saw) in my head with a United shirt on as I identify them with greatness and Manchester United.
Ronaldo I personally see as a great player, one of the greatest in the history of the sport, but I don't think of his greatness at United when I think of him, I think of his raise to greatness at United, and subsequent greatness that from my point of view makes him a Real Madrid legend.
Sure he could be a legend for both clubs, but for me, I feel like there is something lacking.