Short version: As has been said, the "level 50" rule consists mainly of not abusing what powers you have.
Long version: By sticking around at a certain level, and this was much more prevalent when enhancing was unlimited, you could unbalance the game for all those below you, through giving out heals, beneficial spells, or large numbers of enchants. When a high level mort was noticed as being on frequently (i.e. had logged a lot of hours without leveling), and especially when it was noticed that they were helping (too much) the newer players, they would be asked to hero. This is not, even then, something strictly enforced, as everything was still quite polite... after a second request (I believe), there would be a reasonable time limit put on leveling. This rule is in effect, although it has been enforced less and less as the years go on.
