I agree with Tylon ... or is more he agrees with me?
Either way, having the ability to be completely immune to everything, even if it requires a ton of time and work to do it, is kind of... hard to balance the game against. Because you have builders that are growing up with this "I can be immune to everything" mentality, who are then asking, "How can I make this area challenging -for me-."
Which has led to brand-abusing mobiles and otherworld damage. Which is great for people who are fully customed and stuff.... but then you hit the people who aren't and just explode on contact. Look at most, if not all, RPGs--even MMOs--there isn't (normally) the ability to get immunity to everything. Sometimes, sure, immunity to one element at the expense of weakness to another element, or half damage from multiple elements.
And this is usually limited to just one member of your party, or you spread the resistances around.
Most MMOs probably have some sort of scaling resistance, such as 33% resistance to fire for X seconds(skills/spells/whatever). There -should- be a cap on how effective resists can get... sure, it'd still lead to people over-charging resistances to "block" brands(which could be altered to lower the maximum resistance ability rather than the current value to still retain their usefulness... but this would make brands stupid OP for mages if their spell damage isn't looked at. And by extension almost all damage across the board(damage on skills[kick, bash, palmstrike, etc etc etc] needs to actually be toned -up- because it sucks even against low level mobiles).
Resistances are nice, but when they become a required to play.... meh. Without decent resists and some sort of sustain(which is in the form of healshield, mostly) you'll get ripped apart pretty quickly. Without healshield, and manashield/earthbarrier, Arthenon will generally chew a person apart on his melee attacks alone in a few rounds. Unless they're me, then I just cast power heal and lulz. Arthenon's melee damage is actually -good- considering his role as a legendary mobile.
Normal mobiles against a -10 resistance(the resistance to otherworld I normally have because of aura) should not be doing that kind of damage--and they can and do.