The number and frequency of mob spawns encourages and rewards those who act the worst. For an example, I'll use myself.
The infrequent attacks, combined with the limited number of mobs per attack, creates a need for intense competition. If you don't go out of your way to grab everything you can, you're likely to end up with nothing.
The invasion sites encourage people to tag as many mobs as possible with no regard for others. Normal farming isn't like this. Even if spawns are limited, they're not so limited that it's worth being an ass. But the invasion goes a step further. Enter the Shadows of Doom. These are summoned by spending 8 runes and will drop 30 and one of the BoP anti-undead pieces. This means them profitable, if you can get the one you summon.
There are two practices which make it hard to get the shadows. One is AoE spam by others, tagging the shadow first. I think this was fixed last night, but I'm not certain. The other problem is leashing. They won't go more than a certain distance from the center; they'll evade and reset. This can be forced by people trying to steal shadows from those who spent runes to summon them.
In my case, I've learned to never expect to get the shadow I summoned. It always gets taken. So I steal from others. That's right, I do exactly what I just said was an ass thing to do. What else am I to do, spend my time racing from site to site getting nothing? I use turn undead, over and over, until a shadow resets, then I grab it.
While waiting for shadows I've learned how to tag well. First off, if I get there early, I pull as much as I can survive. Once the zerg comes, then I move off tot he edge. Let people fight in the center for scraps. I have exorcism ready, in a convenient hotkey (I actually switch it with CS just for this). I see a spawn, bam, instant tag. I have to do this or else I get nothing.
This is wrong. People should be able to participate in this event without being forced to fight over every single mob, lest they end up with nothing.
 One idea I came up with was to combine the invasion and zombie events. Allow zombie players to get runes from randomly spawned necromatic crystals near or in cities. These would increase their health significantly and run speed slightly as well as slowing the rate at which they decay and die. Killing a zombie with one of these would allow players to then loot the rune, similar to the looting of gold from corpses in BGs. Remove the gear from rare spawns at put it on the vendors for significantly higher prices. The goal is to make the gear possibly take the same time to acquire, but remove the current winners and losers system.