S and p character with hybridisation

A nucleophile will tend to donate electrons which are at the highest energy level. Generally:

Which makes sense. For example, we already expect ammonia to donate its lone pair rather than a hydride ion (H^-). 

There is a related pattern in the hybridisation of s and p orbitals. In ethyne, a carbon atom has two p orbitals used in pi bonding, and one s and one p orbital used in sigma bonding. The sigma bonds are therefore 50% s character.

For a carbon atom in ethene, one p orbital is used in a pi bond and one s and two p orbitals are used in sigma bonds, sigma bonds in ethene are therefore 33% s character.

In ethane, sigma bonds are 25% s character.

A s orbital is lower in energy than a p orbital, so molecular orbitals with high s character are lower in energy.

A visible result of this is the structure of CH₃⁺, which has three sp² orbitals and one empty pi orbital:

If someone asked "why isn't it sp³ hybridized with an empty sp³ orbital. Just remember that lower-energy orbitals only affect stability when electrons are contained in them. sp² hybridisation rather than sp³ in this molecule lowers the energy of the occupied orbitals and increases the energy of the unoccupied orbital.. 

In other words, it won't waste s character on an empty p orbital.

And obviously if you added a lone pair to make CH₃^- then it would sp³ hybridise and adopt a tetrahedral shape.