Minix3 was released at October 2005: 6 years ago. If it were more than a toy, it has had time to mature. I guess it is at about the same point that Linux was when it was six years old (getting support for shared libraries). But Linux was already used for web servers at the time.
And then there's one minor thing about the license: the license of Minix 3 is a four-clause BSD license. This is unlike the 3-caluse or 2-clause BSD license for the bulk of the licenses of the other BSD-s (right?). This puts it in the same boat as OpenSolaris as having a GPL-incompatible license (right?). Something with that vintage of 2005-OSes, I guess.