Before I continue, I must warn you that this treatise will contain spoilers, so if you haven't read the first five books in the trilogy, turn back before it's too late.
My primary concern is that of how there can even be a new Hitchhiker's novel. As I recall, Mostly Harmless ended very finally, with nearly everybody being extremely dead in all dimensions of space, time and probability. It doesn't seem possible to come back from something like that.
Then again, the impossible is actually fairly common on the Hitchhikers' universe(s). Given the infinite size of the universe, the other universes and dimensions, and in fact the whole sort of general mish mash (WSOGMM), nothing can truly be said to be impossible. Don't believe me? Perhaps we can discuss it over breakfast at Milliway's.
Another concern, one which has been voiced ad nausaeum since this book was initially announced, is that of the author himself. The problem that most people seem to have can be summarized thus: "Eoin Colfer is not Douglas Adams." History has shown us time and again that it's the blindingly obvious that is often the most difficult to handle.
Personally, I'm not expecting this new book to be just like the others. As long as this book is a reasonably light, funny tale set in the increasingly bizarre WSOGMM that Adams created back in the seventies, I'm OK with it. Think of it the way one thinks of a Star Trek novel. It doesn't need to be written my the creator to be good.
All told, I'm quite looking forward to reading And Another Thing.... I imagine that it'll be a bit like seeing an old friend after a very long time.