I think perhaps the main issue is one of intention. In other words, upon (re)consideration, I would say that a datacenter is a building or portion of a building constructed or arranged for the purpose of housing servers and connecting to the Internet for exchanging and making available data. Thus, though your basement might meet some of the aforementioned datacenter requirements (high speed connection, multiple servers), it's not a datacenter because it was not intended as such, and in fact also serves other purposes for which it is not only specifically intended, but perhaps also better suited. This definition also allows for datacenters that are obviously datacenters, but might have, for instance, slow uplinks or a lack of security.