Blizzard appeared to have only been testing a simple deathmatchmode, without attempting to throw any objective-based play in the mix like capture the flag, king of the hill or perhaps something more MOBA inspired.
While there will be traditional deathmatch and team deathmatch modes, in Siege mode the size of your Mech correlates with your ability to collect energy.
And though Gearbox has mastered single player campaigns and co-op, Deathmatch has really never been their thing which is a problem considering how heavily Call of Duty relies on the mode.