For my tastes, as guidelines I think are worth sticking to, a keyword mechanic needs to be doing offloading in the rules text that makes things more convenient , it needs to have human-understandable reminder text that isnt just the rules text written out, and it needs to be something that other things can usefully reference
Shock Brigade, the premier Mardu common, is in fact good, and I see these come around way too late
Slivers And Spikes Sliver Swarm is a tricky precon to upgrade because a lot of the most obvious upgrades have spiked like crazy
Its also a staple of most blue Commander decks
Others still find horror in a particular scene from Pee-Wees Big Adventure
Sorcery Clones (CTRL to add secondary) Copy Drivnod, Carnage Dominus If a creature dying causes a triggered ability of a permanent you control to trigger, that ability triggers an additional time