Generally speaking it's as other have said, muds will be noisier, not handle quite as well, and wear faster. To a large extent though it's going to depend on what M/T you go for and how shagged your current tyres are as to how big the difference you will notice. The more extreme muds such as Special Tracks and the like, don't have the best reputation as road tyres and the difference between them and A/T's on the road would be very noticeable. On the other hand, less aggressive muds such as the KM2 are reputedly very good on the road and many people who run them say that they behave more like an A/T on the road (I've not used them myself so can't comment first hand).
Likewise, if your current tyres are well past their best you may not notice much difference. I recently swapped BFG A/T's (with 2 out of the 4 knackered) to Insa Turbo Dakars (the old BFG M/T pattern). The Insa's are noisier, but grip and breaking actually seem better, probably due in part to the Insa's being a softer compound and probably in part the BFG's being near the limit tread wise. Can't say I've noticed any difference in fuel consumption at all. Wear seems minimal with the Insa's at the moment but it's probably fair to say that they won't last as long as the A/T's they replaced.
HTH