A very common and serious problem for people recording and mixing music in home studios is that the recordings don't sound the same on the big CD player/entertainment center as they do on iPod earphones, and different again in the car.
This can be really frustrating when it sounded so great in the room where you mixed the music. I wrote an article about why this so often happens called Your Ears Are Lying to You – Why Your Song Sounds Great in Your Room, But Not in Your Car.
The big culprit is almost always a less-than-ideal acoustic situation in the mixing room. First, these rooms are usually coverted bedrooms, which are notoriously bad, acoustically.
Their boxy shapes with flat and parallel walls create lots of havoc with sound waves, adding energy at some frequencies and removing energy from others. So if you add and subtract volume with EQ, it may sound great in that room, but terrible somewhere else. That's because the EQ was compensating for the bad acoustics in the room.
Another problem for the home recordist is that monitor speakers are usually not that great either. Some folks don't bother with them at all, mixing instead on headphones. Bad idea! Mixing and mastering in headphones is universally frowned upon because music sounds much different through the air than in headphones and again, you get a non-portable mix - one that won't sound the same on different systems.
Likewise using your computer speakers is not a good idea because if they are "good" speakers, they'll have been designed to complement the sound, meaning they make the things sound better than they really are. If they are average computer speakers, they'll simply be incapable of accurately reproducing accurate sound across the spectrum. So you need to invest in a decent pair of monitor speakers.
Lastly, a lack of knowledge about frequencies can create a poor mix. If you have taken care of the stuff above - treated the mixing room or designed an ideal space, and gotten good monitor speakers - most things will take care of themselves and you don't need to be an expert in all the frequency and sound-y science.
However, if you don't or can't have that awesome mixing space, having at least a rudimentary knowledge of that stuff, AND being very aware how the bad space can and will affect your mix, you can take measures to counteract the problems.
In my case, I will make a mix and then take it downstairs with a notepad and write a lot of stuff down when listening on the "big" system. Then I'll take the mix into the car and drive around doing the same thing. Though you should pull over when writing your notes:).
Finally I take all the notes back to the mixing room and make corrections. Then I do the process all over again. Yes this takes longer, but it's worth it, especially if you can't afford to acoustically treat your room or buy good speakers.