Playing with less or more than 10 Kingdom card types in Dominion
I think playing with less would be fine.  From the games I've played with there are about 2-5 stacks that never get any buys, and 1-2 that get bought down really fast.  For inexperienced players I think it would be a big plus.

However, for experienced players it could change some of the strategy a bit.  The other stacks, even if unbought, can serve as a threat that players may buy those.  For example, if there were multiple reaction cards then the attack cards seem less desirable.  So right there 2-4 stacks may be off-limits on the premise of mutually assured destruction.  However, with no reactions attack cards gain a lot of value.

Thus adding more cards would be bad IMHO for experienced and inexperienced players but for different reasons.  The inexperienced players would be too confused while the experienced players would settle on 'sure fire' strategies with so many other strategies to game against.  A limited selection limits the amount of competing strategies, and hence may reduce the number of spoilers to more entertaining card selections.