Traditionally primes always delivered far better image quality than zooms - particularly in terms of resolution and contrast. Today the difference probably isn't quite so clear cut especially when you consider the abilities of the pro spec models - albeit at a cost. Certainly the resolution and image quality of the 28-70 f2.8 Sigma I own are very close to prime and the 10-20 and 17-70 are not far behind. Nevertheless I would still have to say that I think both still fall an appreciable, though not intolerable, degree short of the simple and dirt cheap 50mm f1.8 or 28mm f2.8 primes I own.

If there is an issue with zooms where they are still well behind primes it's in their 'drawing' ability. The optical compromises required for zoom does leave them more vulnerable to distortion issues. Primes are generally easier to optimise for minimum distortion as you are generally dealing with a far tighter focal range - a few mm of focusing rather than tens of mm...

Zooms also tend to be bulky and the faster the bulkier - my 28-70 is enormous compared to the slower variable aperture 28-105 Canon. With modern SLRs this is not generally an issue although personally I find very big lenses on small bodies to be awkward. On the other hand with a system intended to be ultra compact lens bulk could be more contentious. A lack of grip area could make handling with large lenses tricky...