OK
none of these comments are totally accurate. Let me explain.

There are mainly three reasons why engines in stock form should not be revved higher than factory specs, and these reasons are the key as to what needs to be addressed if you want to go higher.

1. RODS
Power remainig constant, Piston to head clearance is chosen based on rod material stretch properties under certain rpm's. Stock rods are more than capable of revving higher, however the more rod stretch, the more likely the piston top is to kiss a valve or head, depending on the clearance. Forged rods are not a neccesity for higher revs if the clearances are adjusted accordingly.

2. Head Valve Train.
Springs & Retainers are chosen such that the spring pressure is just enough to withstand the forces generated by the cam lobe profiles at a max rpm (chosen as a function of rod & crank displacement).
Uneccesarily high spring pressures will accelerate wear on your vales & seats.

3. Engine Geometry
This pertains to the size of the crank & length of the rods.
Engine Deck Height remaining constant, the larger the crank, the smaller the rod length. The opposite holds true also.
So, a shorter rod, larger crank will cause piston speeds to be very large in comparison to the opposite scenario, and since F=ma, the forces on the cylinders are larger with shorter rods.

If you're interested in engine physics and what is required to make an engine rev higher, or produce more torque etc etc, grab yourselves a copy of "Smokey Yunnik's" engine building handbook.

Hope this explanation helps