Challenges
All mole builds must meet several challenges in order to be successful.
1. Getting a Burrow Speed: Naturally, this part of Mole cannot be ignored. This is what makes Mole Mole. For the most part, this comes down to race selection. There are other ways to get a burrow speed, however, that will also be discussed.
2. Getting Spring-Attack: While this is a relatively simple challenge to complete, that doesn't make it any less vital. My own experience on these boards has shown me that people are often not willing to use their valuable feat slots on Dodge and Mobility (and thus, Spring Attack). In addition, Mole needs to have Spring Attack ASAP to be successful at what he does. Other move-attack-move methods (such as Shot on the Run) are also acceptable.
3. Locating Enemies: Mole spends most of his combat time underground where seeing enemies is more than a little difficult. If an enemy moves, he has to know it. Attacking itself is not a problem since he is above the ground when he does that, but knowing where he needs to pop up is another story. Some build will ignore this chllenge, specifically the ranged ones.
4. Flying Enemies: This is an obvious problem for someone who spends so much of their time underground. Not only are they normally out of melee range, but they also are harder to detect while Mole is underground.
5. Earth Elementals and Similar Enemies: No matter how powerful you are, Earth Elementals still rule the ground. They have capabilities I have not yet figured out how to incorporate into a PC by 3.5 rules, and can be quite dangerous to a burrower.
6. Non-burrowing environments: Places where Mole can't disappear into the ground are Mole's Achillies Heel. He can't retreat to safety when on a boat or the Plane of Water and so is no more amazing than any other character. Thus, Mole needs to still be useful in these places.