When I first started listening to music in a big way years ago I always liked to hear that little high end *ting* coming from a good set of tweeters. It made cymbals sound like real cymbals, and you could hear the pluck of acoustic guitar strings along with the finger noise. Later on I realized that a lot of that energy isn't in the high end but some of it is. I still like that effect, although not everyone does. Some people prefer a mix that doesn't have much high end energy in it.
EQ clarity between tracks, is probably one of the best ways to bring out each track and in doing so brings out more in the instruments you want to hear it in,also, like someone else has already said phase cancellation can generally muck things up. Some pre amps/mics can add subtle harmonics and exitation in a good way simply because of the way they are designed, and this can be duplicated pretty well with the right plug ins.
I would rather err by adding a little much than the other extreme and not adding enough, but that's just me. My rational is that most systems are low end systems and won't reproduce it anyway. On the ones that will, if it's too much for the listener they have a tone control to roll some of it off. Nothing is worse than wanting more sheen, rolling your tone control up and it isn't there. I have both the Sonic Maximizer plug and the hardware. In the right setting it can help but the catch is not to use it like catsup at the restaurant.
I agree with others here that less is usually more..even though Alan Parsons said it :) Cutting rather than boosting the right frequencies can clarify a mix.If it is boosted in a desired range the boost is very minimal. Trying to impose a trait to a track that was never there in the first place usually doesn't work. Good at the start means something good to work with. Mediocre at the beginning has usually resulted in me fighting my mixes to death and still not getting good results.
There are some good approaches to a mix with regard to EQ and frequency roll off for various instruments to fit better in a mix, but none of them are across the board every time and this is why I am starting to really dislike those pre dialed settings in things like Alloy 2. Bass adjusted to one song will sit different in another, same with all the rest. Unless you make carbon copy songs with all the same instruments every time recorded in the same space there will always be subtle differences.