Temporal and spatial regulation of gene expression that generates cell lineages comprising interdependent cells with a common genome. Among the metazoans, Porifera have a totipotent and rudimentary stem cell system that still provides mechanisms of development and differentiation. Transdifferentiation remains differentiation and I just see it as the basis of cellular plasticity.

Having dabbled in PDZ domains (before they were named PDZ domains and we just called them discs-large-like domains), I was pleasantly surprised to see recently the discovery of essentially intact post-synaptic density components in a multicellular eukaryote lacking a nervous system — the signalling and scaffolding components were there nonetheless.

Granted, other mechanisms of cell signalling exist that do not rely on animal-style junctions. But the other criteria remain challenging. If we do impose a common genome limitation, which I am convinced is appropriate, then prokaryotes with their relatively widespread propensity toward lateral gene transfer will be even more challenging to accept as anything beyond colonial.