I'm worried that customization might make it hard to tell friend from foe... like in Source the Insurgents tend to wear darker camo while Security has a lighter desert camo, but if you start allowing Security to use even slightly darker camo or slightly change their silhouette, I think that could cause some balance issues.
I remember in Socom, I used to always choose the T model that wore dark brown headgear instead of the green/red stuff, because when I went prone next to a rock or rubble, I'd blend in really well. Half the time, I think people thought my head was a stone and that little bit of hesitation won me many gunfights.
I'm hoping the customization is limited to smaller stuff like beards and gun camo. It would be badass if I could deck out my M16 with PMAG's and other Magpul type stuff (diff colored rail covers, etc) without needing to resort to mods. Also, other players could see my badass-looking M16 without needing said mods. But I'm guessing that opens a can of worms with trademarks and copyright... not long ago Activision was just sued by AM General over their use of Humvees in the CoD franchise.
Being able to toss on some tan skeleton rail covers, that sorta thing I think is acceptable customization without throwing off game balance. Also, beards would be really awesome.