Diazepam is an interesting drug. The question of just how long diazepam works is clouded somewhat by the fact that the main metabolite of diazepam, nordiazepam (the desmethyl metabolite) is both pharmacologically active and has a long half life, of several days. Thus, the recommendation for using diazepam for alcohol withdrawl is sound, as the combination of the drug plus its active metabolites are indeed long-acting. As noted, diazepam — like other benzodiazepines — acts by binding to a unique allosteric site on the GABA receptor. The efficacy of binding, however, isn’t equivalent — lorazepam is more potent than midazolam, which in turn is more potent that diazaepam. Thus, there are pharmacodynamic differences, at least at the level of drug binding, which favour lorazepam’s efficacy.
It should also be noted that a Cochrane review by Appleton et al (Evidence-Based Child Health 2009;4(4):1806-9) notes that lorazepam is associated with equivalent efficacy for the therapy of seizures as diazepam, while having fewer adverse effects.

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