Like most above, I don't think it's which "aspect" of the car is changed, it's how well it's done - and goes with the rest of the car! That's why I didn't really want to vote - but have done so I can see the results. I went for wings:
my car 
)
Paint & Graphics jobs that look like they've been picked by a colourblind bloke with cataracts are sh1te - there've been some classic examples on fleabay recently
Did you all see that 33 on fleabay with a Scooby scoop on the bonnet?

That's what gives car modding a bad name
I don't particular like Veilside "snowplough" front kits, doorstacks are generally a "go faster" chav mod, some wheels are really crap, I don't like carbon boots, etc, etc. But, on the whole, well put together (and not OTT) mods are an improvement and make our cars "individual".
At the end of the day though, beauty is in the eye of the beholder and, like Mook says, it ain't about ugly, it's about sh1te
Matt - my wife says your car is ugly (you can take this up with her if she comes on our Dec NW meet

). I say it looks the business because I know it "does what it says on the tin"

- it's a package.
I'd liken it to an Apache attack helicopter! - Beautiful? No. Gobsmackingly, devastatingly, brutally, impressively AWESOME - yes!
