Several tips: Something that can take a rack. Something with a lockable fork. Avoid dual suspension. Avoid anything too heavy. Unfortunately, these are often cross purposes. Finding a mountain bike that can take a rack for carrying your stuff is difficult. There are a few but they can be hard to find. Jamis XC Sport bikes have rack mounts and I think some of the Treks do. You just have to look. Disc brakes make mounting racks more challenging but not impossible. You just have to be creative.

Lockable forks are fairly common but, if you really want to ride off-road, you also should be looking at higher quality forks. However, those don't usually come on bikes that have rack mounts...that whole cross purposes thing

Weight is also a problem since most of the bikes that can take racks are on the lower end of the bike line and, thus, end up with weightier parts.

Dual suspension can be handy in some situations...works wonders in ice and snow pack.. but it's usually too active for most commuting and you really can't find a dually with rack mounts.

There is nothing wrong with using a mountain bike for commuting. I do it all the time. Yes, it's slower but if you can find an off-road option, it's much more fun than "regular" commuting.