Great, the lance as one-handed weapon or two-handed weapon controversy has already been noted.
The other questions I notice popping up fairly often (and my opinions) are:
1. Do you need to use Handle Animal to direct a mount you're riding to attack, or is it covered by the "attack with mount" option under the Ride skill?
Rules of the Game - All About Mounts (Part 2) indicates that the Ride check is sufficient. However, the RAW are unclear, and some DMs may also require a Handle Animal check.
2. What can you do while riding a mount?
This is covered by Rules of the Game - All About Mounts (Part 3). Generally, you can take a full round of actions - a standard action plus a move action, or a full-round action - even while your mount is moving. However, your mount's movement may restrict or penalize certain actions. For example, you can normally only make a single melee attack after your mount has moved more than 5 feet. If your mount makes a double move or runs, your ranged attacks are penalized, and you have to make a Concentration check to cast a spell.
3. How do I make a Ride-by Attack?
This is addressed in the 3.5e FAQ in the Monsters section (as it references a question on Flyby Attack). Essentially, when using Ride-By Attack, you must move in a straight line towards the closest square from which you can attack your foe and still continue in the straight line of the charge.
4. Where can I find information about alternate paladin mounts?
3.5e DMG p. 204