Drums of War was basically a PvP patch. It introduced Cross-Realm battlegrounds or Battlegroups. This was Blizzard's answer to players having to wait literal hours to get into a battleground because battlegrounds, once upon a time, were relegated to players from the same realm. Although the wait was shortened due to having new players to play with, this was a sad day for me because cross-realms effectively ruined inter-realm, PvP guild rivalries (wanna know how you can tell someone played during vanilla...I told you we'd tell you). Patch 1.12 also introduced World PvP, turning Silithus and Eastern Plaguelands into PvP zones where players would battle to control the land for their respective faction. For those of us who were born and raised on PvE realms, this was pretty exciting and gave us a bit of a taste of PvP-realm life.
Blizzard developers settled on Drums because it was the last patch before Vanilla became TBC. In my personal opinion, Blizzard should use June 2006's Patch 1.11.0: Shadow of the Necropolis as its base. Shadow of the Necropolis was the last time Classic felt like the classic WoW that so many Vanilla players reminisce about. Patch 1.11.0 introduced the last 40-man instance raid, Naxxaramas, the floating necropolis above Eastern Plagueland's Stratholme.