Ah, the early 2000s. It was a time of great upheaval and revolution, particularly for PC graphics.
Almost all at once, game designers could assume players had access to much more powerful 3D rendering technology, and game characters went from looking like weird, half-animated finger puppets to less-weird, more fully-animated finger puppets with faces.
Case in point: My current obsession, No One Lives Forever. The first game came out in November of 2000, the sequel a shade under two years later, in late September of 2002.
I finished the first game over the weekend and have been working my way through the second one this week, and I'm struck by how totally the game was overhauled in such a short amount of time. The games seem like they were released in entirely different eras. (In a way, they were!)
NOLF developer Monolith seems to have had a much larger budget to work with the second time around, but the technological increases also coincide with the broader boom in 3D PC graphics, and the number of gamers that had beefy 3D cards in their PCs. Remember, NOLF 2 came out just 2 years ahead of Half-Life 2, a game that somehow still holds up visually almost ten years later.
For the sake of comparison, here's the opening cinematic from No One Lives Forever:
And here's the opening cinematic from the sequel:
Dang. Big difference. They're emoting with their eyes and body language! They have ears!
The overhauls don't stop at visuals, too—the sequel is a much more robust, "current" feeling game, with an RPG-like leveling system, greatly enhanced stealth (hooray for body-carrying!) and my favorite video-game move of all, the lean. Across the board, the game seems to have had a good deal more behind it than the first game. (Though it already seems that the sequel may lack that certain longwinded charm that the first game had, and I'm already not as into the story as I remember being.)
The closest more recent series to did something similar (that comes to mind) was Mass Effect, with a sizable production gulf between 2007's Mass Effectand 2010's Mass Effect 2. Though those games were separated by a bit more time and still weren't quite as obviously different as the NOLF games. I'd imagine we might never get a period of change as substantial or consistent as the early 2000s, though it'll be interesting to see if the coming generational console shift offers something close. We'll see.