Yes, there's the obvious difference: From 1997 polygons to slick, modern day graphics. But more than that, the character's design has subtly changed over the years. Let's examine those changes.
Via 2ch, here is the the evolution of Cloud's mug. Of course, if game graphics were used for the earlier titles, the alterations would be more even more dramatic, but the point here is to see how the character's conceptions has changed.
Thus game art, it seems, is used for the era when the graphics weren't what they are today.
If you are wondering why the Advent Children Cloud can't be used for the remake, Final Fantasy VII character designer Tetsuya Nomura told Famitsu that it's because the AC model is over ten years old and "too old" to use from a technical standpoint. Fair enough.
And if you are wondering why the Remake Cloud looks somewhat sickly, Nomura also added that the trailer's lighting makes Cloud looks pale and thin. FYI!
Do you have a favourite Cloud? What about a least favourite one?