1. Auras do not make an item a "support item".
2. Will of the Ancients is still very good for mages, it is just closer to its actual value in cost right now.
3. Spell vamp has been found to be problematic in large amounts; therefore, the stat must remain very limited with only a few exceptions in champion kits that include it at the cost of a drawback somewhere else that balances out the effects.
4. Darius was "balanced" when he first came out. He was actually "underpowered" when he first came out in games where all players play competently. By design, Darius mostly stops scaling as soon as he hits level 13 and his damage abilities can no longer be leveled up. After this point he becomes very item dependent, and the lack of defenses/escapes/steroids in his kit coupled with his melee range make the gold and item requirements to keep an advantage in durability and damages astronomical.
Darius will always fall off eventually, and unless the rest of his team is a lot stronger than yours and has racked up a large gold advantage because of Darius, his presence will eventually cost his team the game. Darius is simply too useless later on to justify a champion slot on a team. He is a liability more than a help.