If you're a good circle-strafer and have quick reflexes, then this is the mode for you. Step up to hard mode and get schooled. More enemies, faster enemies, and less health. Insane amounts of enemies make this a challenge for even the best FPS players. Consider this mode for co-op games, but good luck going it alone.
Enemies become invisible, and phase in and out of sight.
This list isn't exhaustive, but there are some things here that you may not have realized or thought about. Be very methodical in how you approach each area. Take a look around and think about your path through an area before the lead starts flying - you won't have time to think later. Listen for enemies appearing, items spawning, and various other cues to let you know what's going on. Save the heavy firepower for those times when you need it.