You can make AddOns to save talent builds. Doesn't get around the respeck cost, but atleast makes applyign talents fast and easy after respeck.
Since Classic doesn't have cross realm grouping (outside of BGs), LFG tool addon can be made. It will use a custom chat channel to send out scripted messages to communicate the group listings.
AoE loot, mount level changes, mana regen and multitaggign are not UI levels stuff and thus cannot be made with AddOns.