Witnesses do two things: 1) sign blocks which keep the blockchain going, 2) decide whether to accept upgrades developed by Steemit. Unofficially, witnesses also: 3) operate seed nodes so nodes can connect to to the network and 4) help promote and develop the ecosystem and platform . 1) and 3) require the ability to manage, operate and maintain high availability and eventually high-capacity IT infrastructure. 4) requires good judgment in deciding how to focus efforts to achieve the best outcomes for the community. 2) requires good judgment along with either understanding of blockchain design and implementation (which encompasses not only the technology but also the economics surrounding it) and/or good judgment in relying on the advice of others if personally lacking in this expertise.

Please do not follow the advice of this post, even though I find some aspects of its analysis to be interesting and significant. Do vote.