In all reactions, you have to take into account the nucleophile, electrophile, solvent, and physical conditions (temperature, pressure, etc)
Most of the time, problems are general enough that we can focus mainly on the electrophile and nucleophile (As long as the solvent and conditions don't interfere with the reaction)
For #SN_2# reactions, we check the nucleophile for its nucleophilicity, basicity, and steric hindrance. If the nucleophile is part of a long molecule, we should also consider potential functional groups that it can interact with
A good nucleophile has an unstable negative charge, but isnt so basic that it would rather abstract a hydrogen rather than attack the electrophile. It should also be less bulky so that it can approach and interact with the electrophile
A good electrophile as a net polar positive moment or charge and is not sterically hindered. For tertiary electrophiles, #SN_2# is virtually impossible.
When it comes to solvents, we would want a nonpolar, aprotic solvent. Nonpolar because we don't want any of the solvent molecules attenuating the nucleophilicity or electrophilicity of the reactants. Aprotic because we want our nucleophile to attack the electrophilic reactant and not a hydrogen