Lighter frames, lighter solid forks, 700 c wheels and tires, slick 32 or 35 mm tires. Better drivetrain since the manufacturer doesn't have to divert money to the suspension fork. Modern hybrids will be lighter, and the tires better suited for commuting on paved surfaces than the knobbies you get on a mountain bike.

Modern mountain bikes are purpose built for technical trail riding. Good suspension forks cost a fair bit of money, as does a drivetrain built for the demands of singletrack. All that costs money. A decent mountain bike costs anywhere from $700 or $800 for entry level and goes up from there.

$400 or $500 is neither here nor there for mountain bikes. You get inferior everything at that price point. However, you can get a quite serviceable hybrid at that price point. Not something you would ride technical trails on, but more than adequate for a 10 mile commute.