Something that most DJs and audiophiles and remixers know about but might not be common knowledge is the
Loudness WarsDo you have CDs that you bought, heard on the radio, then listened to a few times but every time you did it just felt tiring or sounded too loud or weird? It's probably a 'bricked' CD, all the dynamic range compressed and the sound clipping for maximum radio impact (ironic since it compresses it even more) and as arthole commented about a certain set of re-releases it's sadly prevalent in music mastering today. Hard-Fi's first LP is the worst I've had so far, the mastering and sound just seemed so odd it put me off the album.
I have to face it with my mashups even - I can compress it for loudness and dancefloors and listening on iPods, but then it will tear your head off if you're listening elsewhere...it's a hard one. I usually normalize to peak, ie make the sound as loud as it can go without losing dynamic range without clipping, but I also use software to make it 'louder' but usually end up looking at the waveform and using it very sparingly....apart from my KP mash where I wanted it LOUD and didn't really care about the lifetime as it's throwaway.
Here's an example - Beatles 'Somewhere' over the years:

Now upto 90's it's OK - just increasing volume due to the better reproduction available...but the 2000s? See how the waveform is starting to clip at the top, which means the louder bits start to lose more frequency and 'punch' or texture. And I've seen waveforms that are totally green - ie mastered to 0db with no peaks or troughs, no quiet or loud bits. Ouch.
Turn Me Up are responding to this, sadly I think it'd be seen as some old gits moaning about the sound but as someone who listens to a lot of music I know that albums that are mastered like this don't stay in my playlist long, it's too painful. Also audibly very clipped/distorted tracks don't even get that far, they get deleted...apparently the new Metallica album suffers from this, in official MP3 form at least. Ouchies.
Here's another good demo of the problem: