The biggest problem is over processing on the 2-bus. when you slamm your mix through a compressor and eq and limiter, you kind of lock it in and flatten it out and that leaves little room to correct anything that might be wrong. Most of the time this is done in less than ideal monitoring conditions and less than ideal equipment so there will be problems. Once you crank that eq on the 2-bus, it's nearly impossible to undo it. if the kick and snare has been squashed into the mix, it's really hard to bring it back out. Spend more time on the mix and less time on the 2-bus. so to answer your question, don't make the mix on your two bus, make it in the mixer.

What is a 2-bus? is that the master send?

It's your resulting 2-channel (L/R) that's produced after you do your final bounce out of your software/mixing console

Aside from what Michael has posted, the other major problem I face is uncontrolled sibilence: those nasty T's & S's that sound really harsh. Those are so much more easily controlled by watching the vocal tracks & high hats in the mix than dealing with them as a complete mix during mastering.

Also mixes that have not been listened to anywhere but one mix room. These sometimes have "tunnel vision": problems that are major & obviously would be heard in other playback systems.

All the mastering that I have had done has been for vinly only - the last engineer I used told me me sibilance was a pain to fix on my masters.

Also I was once told in a mastering session that there were 'phase problems' with a bass sound I used. He said the needs to sit dead center in a stereo mix - some spatial enhancers (like stereo widening effects) can mess it up if its not mono.

I used to put my tracks thru soundforge & use the 'increase volume' controll to make snares and kicks hit the rails. They sounded tight played loud in the studio and compared well with the clients 'brought this to compare it' cd - but when I listened at a more moderate level on a masteing engineers system I could hear the tiny fizz of distortion on cymbals & bass/kick drum notes. I dont do that anymore...

I hate the overly bright vocal and underly bright or dark snare that seems common in electronic styled music.
So many times I hear this. arrrrrgh!!!

Or the Huge sub build up that is a terror to deal with

and the premastered t-raxxed tracks that come in squished as fcuk with no impact and no definition.

I work for new/unreleased clients who expect to leave the studio with a finished product and need the production on their tracks to sound comparable to that of the genre. For R&B HipHip this often means loud bright and punchy - but mostly just LOUD. To get this you will need a reduction in dynamic range and some distortion - however you look at it.

As a producer I can imagine this process as the 'icing on the cake'.

As a client this must seem like a major confidence boost for when they present their product to A&R - which is nearly always done before the final mastering

As a mastering engineer this is an extra headache - having to undo the damage before starting to enhance the mix.

So now I give two mixes. one for their confidence - to play, one to take to mastering.

I work for new/unreleased clients who expect to leave the studio with a finished product and need the production on their tracks to sound comparable to that of the genre. For R&B HipHip this often means loud bright and punchy - but mostly just LOUD. To get this you will need a reduction in dynamic range and some distortion - however you look at it.

As a producer I try to imagine this process as the 'icing on the cake' but its really just to keep the client happy.

As a client this must seem like a major confidence boost for when they present their product to A&R - which is nearly always done before the final mastering

As a mastering engineer this is an extra headache - having to undo the damage before starting to enhance the mix.

So now I give two mixes. one for their confidence - to play, one to take to mastering.