It drives me absolutely nuts to hear people dismissively complain that a follow-up game that uses the same engine and some of the same graphics as the original “looks like a mod.” Lately we've heard it about Left 4 Dead 2 and BioShock 2, and we're hearing it now about Fallout: New Vegas (which comes after the whining that Fallout 3 is a mod of Oblivion).
I just don't get it.
If you're a total graphics whore, sure—I suppose that anything that doesn't look significantly better than what came before is unacceptable and under no circumstances worth paying $50 to play. Me? I'm more of a gameplay-first kind of guy. As long as a game isn't hideously ugly, I'm more concerned with how it plays than how it looks. If the last game in the series was good enough that it left me wanting more, then I couldn't be happier with a load of well-made new content using the same engine and graphics, as long as it brings something new to the table in terms of setting, story, enemies and items.
To use classic examples, Fallout 2 did it right by adding an entirely new story, a new map, new weapons and armor, new enemies, and UI improvements to the engine of Fallout 1. Star Wars: Knights of The Old Republic II - The Sith Lords was doing really well by adding a new story and characters, new planets, new weapons and armor, etc, right up until the disastrous ending. Playing these games is a different experience from their predecessors, even though they're similar in gameplay and appearance. They're new adventures.
By contrast, X-COM: Terror From The Deep did it wrong by simply re-skinning and renaming everything with a cheesy underwater theme, (just a handful of aliens and techs didn't have direct equivalents in the first game) and adding only a few UI improvements and the dreaded two-stage terror missions and alien base attacks. It really was the same game with a different look, and that wasn't different enough to call it a new game.
I think what really offends me about making the comparison between creating a new game by building on an old game and making a mod is that it makes it sound as if mods are a bad thing. Hey . Mods are awesome. I'd be completely willing to pay for a great total conversion mod—and don't kid yourself, the only reason we don't have to pay for community-made mods is that mod teams legally can't charge you for them (at least, not until Blizzard launches the planned StarCraft II content store). A lot of hard work goes into creating that content, which is on occasion as good as or better than what was in the original game. It doesn't make any sense to diminish it by implying that it's not worth paying for just because it only looks as good as a past year's hit game.