How exactly do you make synthesized music sound 'real'?

Well I use some very 'real' sounding Sound Fonts, but even with VST's like reverb and stuff, I can't seem to make the music sound real enough. What exactly makes synthesized music sound 'real'? Is it the balance between instruments? Is it the reverb and stuff? How can I make my own personal creations sound real? I'd like advice on that.

sf2 are seldom realistic enough. More realistic are big samples and high-quality synthesizers (if you want electronic sound, not orchestral instruments).

As Jean-Michel Jarre says, if the music sounds without heart, it is not because of the machine but because of the human being in front of the machine. It depends on what you want to achieve and what you have to communicate.

The balance between instruments is also important as well as the part of the spectrum that each sound occupies. Too many sounds which overlap don't sound good. This is the craft of mixing and mastering engineers.

The most important thing is the basic realization of the idea to sound good. If something is good, it will sound OK even at .mid stage; the idea should sound good with few instruments. If this is not achieved, then adding more and more effects and parameters usually won't make it sound very better; sometimes it could even worsen it. So, while layering, so to speak, look for every layer to be finished and stable enough before adding the next.

Well, in my opinion, with top notch software and plenty of time and pacience, you can make a synth track sound 99% real. You have to be very expirimental though, which now I'm starting to doubt that many can pull it off. Its best striving for atleast 60% real.

Just know that real recordings are editted, and theyre all recorded differently. So some factors depend on sound quantity vs clarity.

You can't :) You can make it realistic, but not real :P

So far, the best attempt at a score which was played by a computer and not by an orchestra in a quite satisfying way was Klaus Baudelt's score for Pirates of the Carribbean (which was all played by computer software and not real players). The reason why it doesn't sound realistic is because real instruments are real. Real instruments are affected and affect the total sound in terms of placement in the room, the shape, construction material and size of the room, the players themselves, and there is the whole issue with harmonics, most of which, although barely audible, are cut in the pre-recorded samples that are used by soundfonts such as the GPO. Also, instruments interact with each another (if you have a violin play a loud G, and you have a few C keys silently depressed on the piano, you will hear the harmonics ringing in the piano), something which cannot be simulated easily (or at all, I guess).

Aside from the differences between the actual sound of real instruments and computer samples (which previous posters have covered quiet well), musicians use a score as more of a guide to what they play and add subtleties of interpretation which make the scored notes more expressive and musical. If the time is taken to put in many many tempo and dynamic changes, and slurs for phrasing, and so on, computer music can be quite expressive.

Adding reverb and effects can make computer generated music more artificial although it depends on the composition - a little reverb is usually good but it can be added as a last step as it won't correct for any deficiencies.