Same here guys, main problems in mixes brought to us to master:
1. too much low end, uneven lows end, flabby, muddy, rumbly low end.
2. clipped mix levels, or at least too hot, or compressed.
3. sibilance, hi-hats, tambourines.
4. muddy, murky, cloudy, or dare I say, tubby upper bass/low mids.
5. mixes with digititis
6. uneven L/R balance
7. phase problems
and last but not least...
8. Vocals too low/high
But Hey, I'm not complaining, I enjoy fixin' this stuff, it's what we MEs do!