1. NO SUSPENSION. you may have to look around for a while before you can find a decent pre-built MTB without front shocks, but it's worth it. cheap shocks simply add weight and reduce structural integrity and good shocks will not be a good return on investment for the sort of riding you will be doing.
2. try and get a frame that has enough rear clearance that you could theoretically switch for 700c wheels later on. If you do end up doing any long distance riding, the extra centimeters of wheel diameter will make a huge difference. although, if you do switch later you will have to either do without rear brakes (which isn't that big of a deal, really) or go disc, which means that you might want to look for a frame that, even if not equipped with discs, has disc mounts.
3. get them to switch the straight bars for drops or time trials. MTB handlebars are designed for twitch control and are death to the wrists on long rides.
4. put semi-slick tires on it. semi-slick are a godsend for primarily city bikes that need to rough it from time to time. though you may want to keep the knobbies as well, in case you ever decide to do some actual off-roading.

or, alternately, you may want to look into cyclocross bikes, but it seems that they are generally priced higher than equivalent quality MTBs. prolly just economy of scale.

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_{I don't think anyone's ever really died from smoking. --ni}