Where do people get the idea that obstacle courses are necessary for custom servers? They aren't even enjoyable!
What would I like to see? Core modifications, kick ass scripted items, and things you wouldnt find in ANY other server. What do I mean? Take the ZombieX mod for Cod5. I'm not suggesting you make a zombie mod for your server, I'm just saying you should make something original (albeit cod5 did already had singleplayer zombies :P)
What I've done (and never brought to light):
Rage mode - Every 10th critical hit triggers rage mode, which increases damage and speed.
Lifesteal weapons - Weapons automatically refund a small portion of health per hit (based on damage)
Mana restoration - Same concept, but spells restore small amounts of mana