Alkali metal enolates
I'll assume we're talking about aldol reactions, ie. the electrophile is a aldehyde, or less commonly, a ketone. Being a more strongly coordinating cation, lithium can actually coordinate to both the enolate oxygen atom AND the electrophile oxygen. This serves to "pre-organise" both reactants for a direct addition via a six-membered, cyclic transition state. The favoured transition states are often chair-like. This is known as the Zimmerman-Traxler model.



The model explains diastereselectivity when that is possible, ie. E enolates give predominantly anti products, while Z enolates give mainly syn products. 

Lithium also stabilises the initial aldolate product, by chelation via both oxygens.



The more highly dissociated Na and K cations presumably don't greatly facilitate reaction via a cyclic transition state, nor form a stable chelate of the product. The reaction is also more likely to be reversible, and the products of conjugate addition more stable.

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