Valve hired on DOTA developer Icefrog to develop a new DOTA product from the ground up in house. Other DOTA developers went off to form Riot Games, which makes the incredibly popular League of Legends. And even as Riot tries to shift the nomenclature from DOTA to MOBA, the community that started it all is still winning out. Even Valve head honcho Gabe Newell said he didn't like the DOTA or MOBA acronym, instead substituting ARTS, or Action Real Time Strategy, in its place.
Filing an opposition does not necessarily mean that Blizzard wants to trademark DOTA -- it doesn't. Rather, an opposition makes light of information otherwise not seen and shows that there is more at stake and more people and parties have a stake in the word DOTA as being a community-owned term.
Valve and Gabe Newell responded to Blizzard's opposition by stating that the game being developed was a true sequel to DOTA and rightfully should have the moniker trademarked. However, the DOTA genre is still very much a term used to describe the three-lane tower setup of the classic DOTA map.
Blizzard will be releasing its own Blizzard DOTA game in the future through its brand new Battle.net Arcade system.