I was hoping you'd say you just don't like how it sounds. I can tell you don't get it since the name Valhalla works on many levels with the multiplayer map. Take a peek at the
wikipedia page for some clues.
1. "Valhalla (from Old Norse Valhöll "hall of the slain"[1])", the Hall of the Slain in Norse mythology sounds very appropriate for a multiplayer map where Spartan warriors kill each other over and over again.
2. "In Norse mythology, Valhalla...where those that die in combat go upon death, led by valkyries." So it's not just a Viking heaven, but an after-life where warriors go to battle once again for the god Odin in the ultimate battle for the gods Ragnarok. Notice a theme of warriors, battling again even after death, maybe a little like respawning ;-).
3. It works because Valhalla the map is beautiful in an eerily similar way to the Valhalla described in Norse mythology and shown in classical art (see example below).
4. Norse mythology is very relevant to the Halo universe, there are references to it all throughout the games and the fiction. MJOLNIR (as in the armor MC wears) = Thor's Hammer, Thor = son of Odin, Odin = Host of an afterlife in a beautiful place named Valhalla.
So perhaps you would have just preferred Bungie's internal name River World, which is an OK name referencing an apparent characteristic about the map. I continue to really appreciate the attention to detail Bungie takes in crafting their universe. I know that the references they make to cultures, mythology and religions all over the world connects the universe more to the people who enjoy the games. It's a very distinct style which gives the series a unique flavor.
I'd encourage you to pay more attention to the details because I think you'll get a lot more out of the story. So many people skim right over these details which is a shame. This is one of the reasons we'll be looking into more of these details at
Ascendant Justice.