I spent years looking for the "one thing" that was making my tracks sound not-quite-right compared to professional tracks. Eventually I realized it wasn't one thing, it was EVERYTHING.
In other words, what makes professional tracks sound good is having well-chosen sounds that compliment each other, mixed with just the right levels, panned and stereo-processed just right, with saturation and compression applied in the right amounts, carefully EQ'd to sound good and not interfere with each other and to fill the frequency spectrum in the right way, run through just the right reverb and other effects, and then mastered with just the right subtle effects, limiters, and compressors.
I'm still working on half of those but I'm getting close. If you post a track I'd be happy to take a listen too.
Another thing I've learned about professional tracks--whenever I see one I notice they have some kind of effects on EVERY channel, and automation of some sort on almost every channel. I'm not saying you should add those arbitrarily, but be aware that it's that kind of fine-tuning that really makes a difference.
Oh, and general rule of thumb for panning: the higher the frequency, the more you should pan or widen it.