Instead of saying that these be the 'underlying direct object', or making the distinction between D / S structure, or an 'unaccusative verb' (presumably defined as a Verb that cannot take an Object, which is, par excellence, of accusative case [abstract case]), I think it will be useful here to talk of the Subject being an 'Experiencer' --- the entity that experiences some (psychological) state expressed by the Predicate (the Verb) (p49, Haegeman, 1996). This may sound more palatable as a definition anyway.
BTW, it is an interesting thing to observe that most such verbs are actually ergative (ie. they are either transitive or intransitive: try with passives)