As you pointed out, the most significant aspect that differentiates a Mars lander from a lunar lander is the presence of an atmosphere at Mars. Developing the appropriate reentry technology and perhaps aerocapture techniques would be vital to a Mars mission.

However, that's just one aspect to landing humans on Mars and returning them safely. As important as the heat shielding and landing system are all the other systems that make up a manned spacecraft. There are a lot of other technologies and techniques that need to be developed and tested, such as long duration life support systems that don't need replenishment, space suits suitable for months of use, EVA procedures to avoid contaimination, as well as avionics, guidance, propulsion, electrical systems, etc. These technologies and techniques could be tested on the moon.

Take the space suit as but one example. The Apollo lunar suits were far too heavy and fragile for use on Mars. We'll need to develop lighter suits to account for the higher gravity on Mars. They'll also have to be more rugged to stand up to months of use in a harsh, dusty environment. Ideally, they'd allow greater flexiblity for the wearer, too. Other than Mars itself, where better than the moon to test such a suit?

Avoiding contaimination is a two-way problem. We certainly don't want to contaiminate the Martian surface, especially as we go looking for life. That would be like pissing in the test tube. At the same time, we don't want to bring outside contaiminates into the spacecraft after an EVA (a big problem on Apollo). Not only is the dust a pain when it somes to systems (clogs air filters and can cause other hardware problems), there's the admittedly remote possibility that you'd bring in something unpleasant or unhealthy. The Mars Society is supposedly testing such techniques up in their practice area, but we need to go way beyond what they're capable of doing.