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I read the Wikipedia article on Eucalypteae and it doesn't actually say what these plants have in common.
(I am now reading through https://bsapubs.onlinelibrary.wiley.com/doi/pdf/10.3732/ajb.1200025 and I think I've found the answer to this question.)
It's a slightly tough question that can't be answered with a simple "they share X characteristic in common". It's more that some of them share a characteristic, and some of those share a characteristic with another load of species within the same sub-clade, and some of the characteristics they have are shared with some more... etc. The American Journal of Botany puts it:
(Synapomorphy: a shared trait believed to have been present in a common ancestor.)
Currently, no unambiguous morphological synapomorphies have been confidently identified as defining the Eucalypteae in comparison to other Myrtaceae (however, see Wilson et al., 2001 , for some possibilities). Within the Eucalypteae, however, subclades are defined by synapomorphies. For instance, Angophora Cav., Corymbia , and Eucalyptus share several possible synapomorphies, such as presence of oil glands that protrude above the surface of the epidermis (“emergent oil glands” of Ladiges, 1984 ).
These are modified into distinctive bristle glands in Angophora and Corymbia ( Johnson, 1972 ; Ladiges, 1984 ), which are a unique synapomorphy for the Angophora - Corymbia clade ( Ladiges et al., 1995 ; Wilson et al., 2001 ). Angophora , Corymbia , and Eucalyptus are also characterized by having compound petals and, in Eucalyptus, a portion of the compound petal is usually modified into a staminophore, or stamen-bearing tissue ( Drinnan and Ladiges, 1988 , 1989a , b , 1991a ).
Other characters are apomorphic but are likely homoplasious, having arisen more than once within Eucalypteae. For example, some Corymbia and Eucalyptus species are characterized by the presence of calycine and/or corolline opercula, or cap-like perianth whorls that are deciduous from the flower at or prior to anthesis (e.g., Johnson, 1972 ; Hill and Johnson, 1995 ; Boland et al., 2006 ). Opercula likely have separate origins in these genera ( Ladiges et al., 1995 ; Parra-O. et al., 2009 ). A caducous calyx that usually leaves a scar on the flower bud (see Johnson, 1972 ; Boland et al., 2006 ) is also an apomorphic but homoplastic character within Eucalypteae, being a synapomorphy for a clade within Corymbia ( Ladiges et al., 1995 ; Parra-O. et al., 2009 ) and having arisen at least once on the stem of Eucalyptus subgenus Symphyomyrtus ( Fig. 1B ).
