Seems to me that what Linus and other Microkernel detractors are saying is that they are harder to write and slower than monolithic kernels. It would seem that many Microkernel supporters also agree with this.

Where they differ is about whether there are advantages to Microkernels that outweigh these disadvantages. Linus seems to think not. Others disagree.

It is interesting to note what has happened to the Hurd as an example of a real world Microkernel project. It appears to have a reached a resonable level of maturity, enough for Debian to build a distro based on it - but is now stalled, despite apparently have come so close to its goal. It would appear that certain insurmountable problems (speed and reliability?) have forced developers to rethink and look at other kernels (L4, Coyotos).

Still the basic idea of a Micro kernel approach is very appealing. I hope someone eventually brings it to fruition (outside of research institutes) and into the real world.

No doubt Linus hopes thay won't.