Good feedback, nice to see it all. To try and reply to some of it:
Permenancy: I could see adding to the length it lasts, perhaps, but I don't think it should be truely permenant. A ranger's pet should have some advantages over a necro's golem, and vice versa. For instance, I don't think that a golem should be able to increase their level at all - it's still an undead creation, not a natural living creature, after all. Pets are living, and capable of learning, thus leveling up, whereas a golem is pretty much programmed at creation with everything it will ever know.
Drawbacks: I wouldn't be opposed to an additional penalty to the necro, but this sort of thing (along with the length of the minion's life) should be used by developers basically for balance purposes. I've already included what I think to be a decent penalty in the fact that it requires perhaps more preparation to pull off than any other skill, uses up an Elite Skill slot, and since, as written, it costs 25 mana per cast, and requires 4-6 casts to end up with a golem, that's talking an investment of 100-150 mana over time to get your creature up and going.
Different types of golems: I sort of like your idea of flesh/bone/spirit, but I think it either aready exists in the fact that a golem's nature is determined by it's components, or can be rolled into it. If you create a glem using parts from 4 spell casters and 1 brawlers, you get a golem that likes to use magical attacks and is a bit more resistant to magic than physical attacks. Maybe by including parts harvest from shelled creatures, you can add armor to it in lieu of more attack strength.
Strength: I'm thinking that this golem should be at least as useful as a henchman avalable at that level, if not as strong. While bone minions/horrors/fiends are all about attack, golems need not be - imagine a golem that's more about protecting it's master than anything else, casting heals and protective monk spells. Variety is one of this beast's strengths.
PVP: See also the last paragraph. I imagine a golem's appearance will vary based on component, but it's appearance won't necessarily give away it's nature. Therefore, what the other team passes off as just another minion/henchmen in order to rush the monk, may actually be pulling duty as the team's healer. Or it may end up lurking in the back lines casting it's mesmer spells, or... basically, the only way to tell what this thing an do is to observe it. No, it won't have as much impact as another player, but it may be enoug to turn the tide of battle. I'm not trying to create a juggernaut minion, just a useful one, that's worth the investment.
Assigned Attribute: I think it may be best tied to Death Magic, though a valid argument could be made for Blood Magic as well. I can also see the necro getting a slight bonus to mana gain from kills the golem had a hand in, as opposed to "ordinary" deaths on the battlefield. Also, how should rank of the atttribute effect the skill? Should it lengthen the lifetime of the golem? Increase the theoretical level of the golem (though this would still be limited by component used in creation)?
Death, Necro: Personally, upon the Necro's death, I think the golem should either go uncontrolled until the necro's revival, or perhaps stand guard over the body, attacking enemies that near the body. the latter is more for flavor than anything. See also notes on Golem Behaviour later.
Death, Golem: Here's one I don't know about. If the golem dies from anything besides "end of lifetime", should the necro be able to infuse it with life again? I'm thinking maybe, though it too would of course suffer a Death Penalty. I'm also of a mind that it shouldn't be able to be resurrected, or at perhaps only brought to life a certain number of times, or having it's resurrection halve it's remaining lifetime. here's one I really want feedback on.
Golem's nature: I think it should act like it hasa very rudimentary intelligence (think a la certain representations of Frankenstein's Monster), and fiercely loyal to it's father/mother/creator. As such, it will act to protect it's master first, and is more likely to heal it's master before it heals another teammate that may be hurt worse. Likely, if it's master comes under a certain level of attack, it might break off it's current offensive to attack it's maser's attacker. In other words, though useful to the team, it should e more useful to the necro.
Bonds: I can see there being a ond represented by in game mechanics between necro and golem. If the golem dies, perhaps the necro's max health drops for 30 seconds. Possible side benefit: If the master is a N/W, perhaps a golem's death fills the master's adrenaline skills. Eh, maybe not.
Some Summary: My goal is not just to create an uber-minion, or a new toy, but to introduce a new mechanic that can be used as an extremely versatile tool to influence gameplay in new and interesting ways.
Anyways, please continue discussion, I look forward to what else you might add.