Well, I preferred running a character, and only DM'ed once or twice, and with hindsight (both from those occasions and from watching games as I played), I can pinpoint some skills that are very different.
A writer wants to box his character in, whittling away his options, until he must undertake the drastic actions required.
In a DM, this is known as rail-roading.
And a writer can set up one and only one solution to the problem, and make it a puzzle because he knows he can endow the characters with the wit to find it. The same can not be said for PCs. (I have known players to sulk at the presentation of a puzzle and not even try to solve it, expecting a clue to fall in their laps.)
Similarly with magic. PCs can swim in magic items when that would swamp a novel character with clutter. But as a writer, you can endow the character with only that which is necessary. ("Necessary" covering a multitude of needs. Mood is sometimes a necessity.) But what your PCs find necessary depends on their own native wits. (I once read a gaming article who thought the multiplication of items had to be a flaw in RPGs, because it wouldn't happen in a novel. Different medium, different rules.)
Plus, the players identify with their characters. You don't have to work up sympathy for them. You can't do that in a novel. You can then endow them with characterization to do it, too, which is the players' providence in games, and many a player has a thin character you will never get away with.
And the game will have three, four -- eight -- characters all of whom deserve focus. It is a rare novel that manages to pull of the trick of having more than one focal character. Other characters can be important, but usually you know who the main mover and shaker is.
And only those characters are the focus. A secondary character can steal the scene in a novel. But players don't want to see the NPCs doing all the fun, or cool, or spectacular stuff.