First thing to be clear on is that remastering and the "loudness wars" syndrome are intrinsically separate things (as a couple of posters have suggested). However it is certainly possible that a remastering could be done on any recording that would produce a result typical of a modern "loudness wars" product.
Consider a 1970s rock recording (something like Deep Purple's Machine Head). When it was initially mastered for LP the engineers had to do certain things to the performance as it was taped so that it could be playable on LP.
First, the volume of the bass had to be reduced. This made the grooves narrower so that 46 minutes would fit onto the space available on the LP surface.
Secondly, the overall dynamic range was closed down. The loud parts were quietened down. Softer parts were made louder. LP reproduction can't handle a wide dynamic range (before analog fans start screaming, this was considered incontestably true in the 1970s).
The treble was enhanced because such LPs were often played on cheap single driver players and their lousy stylii and speakers wouldn't pass much of the treble that was on the record to the listener. (If it were a sixties recording it would be enhanced even more.)
Finally they didn't worry at all about high frequency noise/hash/hiss etc because none would be evident when the record was played.
When CD arrived in the early 1980s these albums were rushed onto CD. This usually meant grabbing the LP master and just doing a digital version from it. Result - muddy, compressed performances with no bass, no dynamics and noisy, glary treble.
So a good modern remastering is the fix for most of that. Ideally, where the tapes are available, they can go right back to the original 24 or 36 track components of the performance and mix again to take advantage of digital's greater dynamic range and relative lack of time restrictions. Technically, this is a remix as well as a remaster.
Of course, the result varies pretty widely. Some (eg King Crimson, Yes, Deep Purple and Focus) remasters are good to very good. Some are so little changed you wonder why they bothered (step forward Rainbow).