By phasing issues, I'm referring to how phase cancellation happens more audibly when you are mixing in mono. If you take two identical sine waves and overlay them spot on (in phase), they will turn out twice as loud due to the "Principle of Superposition". Assuming an x axis at y = 0, as you shift one sine wave to the right, there is a progressively increasing phase shift, and what you get is a progressively increasing phase cancellation between the negative and positive values of each sine wave.
Figure 1. Two sine waves overlapping, at a phase shift greater than 0.
This phasing may be considered an issue when certain qualities of your sounds are made less audible because they partially cancel out, and the phasing is more audible when everything is in one channel (mono) than when everything is spread out in two channels (stereo). This phasing sounds like you're pushing down on your sound at potentially irregular moments in time. This may be hard to imagine, but let's say you drew out two identical sine waves on identically-sized pieces of paper (or you could actually do it in real life if that helps). Basically, what you're effectively doing in stereo in real life is taking those pieces of paper and aligning them approximately diagonally, in a sense, and they are approaching your ears at, let's say, 60 degree angles. So, they have less direct phase cancellation and the effects of that are less audible.
This picture below is like a top view, where the blue lines are the horizontal edges of the pieces of paper, or the top of your sine waves.
Figure 2. A typical speaker setup in a home listening area.
You might find it easier if you try playing around with this Excel sheet:
https://app.box.com/s/fxbkgaxkodku2nmsk52oj7i5lotyvncc (note that 10^-16 is close enough to 0)