The appearance of the map, the music, the monsters and the items all have an impact on eachother. That is, they're all interlinked and can pull eachother up or drag eachother down depending on which is which.
If you have a map that looks crap and has bad music, you can still pull it off if the gameplay is great. Likewise, a visually impressive map will help smooth over gameplay problems. For me at least, music is also a real boost.
It's not just this countering feature that affects maps. Multiple things on the same general level will stack on eachother. So a plain map with bad weapon placement is considerably worse than a plain map with good weapons, or an okay-looking map with bad weapons. The whole is greater than the sum of the parts.
Originality also comes into play; some ideas have been gone over too often, and some ideas were avoided for a reason. The overall theme of the map can pull it down if it seems too generic. I'm pretty sure everyone has played a linear or 'tunnel' map for instance.
What I'm trying to say here is that a plain, square room filled with Krall is lame

Try smaller rooms, or less Krall. Maybe add terrain features to make the fight more interesting. That many Krall will invariably cause some damage, so some scattered health packs might not be a bad idea. Pillars or just some cubes lying around will help block fire and turn it into more of a hide and seek, while some dangerous terrain or staircases will give the players something to consider.
In my opinion, any room that big should be made into a proper arena
