So, the ontological argument in its ancient form is very much vulnerable to the criticism you gave. When I encountered it in philosophy 101, it ran like roughly this:

(1) A being that exists is greater than a being that doesn't exist.

(2) Therefore, the greatest possible being, God himself, must exist.

We talked about it for a while, and then the prof rephrased it with "vacation spot", and we all laughed and moved on. It's a well-known critique, and it shows the argument must break down somewhere. We stopped there (as you did), but I think it's instructive to see why the argument doesn't work.

I came up with what I thought was the core problem: the argument doesn't prove what you think it does. It doesn't prove that God has to exist, only that your concept of him must include existence if it's to be coherent. For example, suppose I am at work fantasizing about the great boss I could have instead of the one I do have. For the idea to work, I must imagine that he really exists, and really works where I do, and is really in charge of my work. Those things are part of the concept. That doesn't mean they're true.

By contrast, when I imagine Mickey Mouse, I don't imagine that he really exists. "Is fiction" is one of the properties of the concept, and I'm okay with that. I don't need to imagine I can look him up in a phone book in order to enjoy stories about him.

So much for the logical.

There's a deeper problem with the argument, though, and it's this: I find it rather amusing that the philosopher thinks the best deity is well defined. I mean, has he ever tried, in practice, to define a best anything? When real people talk about real things, they often have a hard time agreeing on the best football team this year, or the best local chinese restaurant. The idea of a "best" or "greatest" something is incoherent in a vacuum. Best for what? Greatest in whose opinion?

Of course the philosopher might say, "No problem. The greatest possible being is the one everyone would agree is the greatest." I think it's rather optimistic of him to so blithely assume that's even logically impossible.

Imagine there was a sympathetic atheist out there. In his opinion, it's a very good thing that God doesn't exist, as his activity would make robust science so difficult -- and a world that has robust science is a much cooler than a world that has a deity. But he's sympathetic. He can see that people who believe in God often have better lives because of it. Some of those born again Christians really do turn things around, so the concept has to be good for something.

From his perspective, the best possible deity is a fictional one.

That rather explodes the argument, doesn't it?

Now, if you read Plantinga in the original (http://mind.ucsd.edu/syllabi/02-03/01w/readings/plantinga.ht...), he gives a long and entertaining summary of the ontological argument throughout history, famous criticisms and objections and his own attempt at repairing it. I cannot claim to have fully wrestled with it (I'm going to be chewing on it all morning, likely -- I hadn't read it closely before, thinking ontological arguments were inherently uninteresting sophistry), but here is what I think I do understand:

The argument runs this way:

(Definition 1) Call a being maximally excellent in a given world W if, in W, it is omniscient, omnipotent, and wholly good. (Note that the omniscience thing implies existence).

(Definition 2) Call a being maximally great if it were maximally excellent in every possible world.

(Premise) There is a possible world in which maximal greatness is exemplified. That is, in some (maybe imaginary but nonetheless coherent) world, a maximally great being exists.

(Conclusion) A being that existed in every possible world would have to be logically necessary.

(Conclusion) Such a being exists.

How does this differ from the ontological argument of yore, and what do I make of it?

I don't think it's trying to accomplish the same things, and I don't think it suffers from the same criticisms.

At a high level, it has a premise -- and once you unpack it all the way, that premise is equivalent to saying God is logically necessary. It's a premise. You're free to reject it. Plantinga claims to accept it (!), but notes that not everyone does (a heck of an understatement). He doesn't seem to care: the object of the discussion isn't to establish that God exists. The article opens and closes stating that the argument will not change anyone's mind, and that the reason we're even talking about it is because the discussion teaches us some interesting things about how to do philosophy -- things like, what do we mean by "exist" and "necessary" and "possible".

What Plantinga does claim the argument shows is that such a being must be logically necessary or logically impossible. God (so defined) is either essential or incoherent. I agree.

It handles my metric objections pretty well -- there's no subjectiveness in the words, all the definitions are nailed down pretty well.

How does it do with cheeseburgers? Let's try it.

(Definition 1) Call a cheeseburger all-yummy in a world W if all beings in W who taste it consider it to be clearly the yummiest cheeseburger they've ever had.

(Definition 2) Call a cheeseburger maximally yummy if it is all-yummy in all possible worlds.

(Premise) There is a possible world in which maximal yumminess is exemplified. That is, in some (possibly imaginary but nonetheless coherent) world, a maximally yummy cheeseburger exists.

(Conclusion) A cheeseburger which is all-yummy in every possible world must be logically necessary.

(Conclusion) There is an all-yummy cheeseburger in this world.

Now, I would say the argument is similarly valid. It establishes that the concept of a maximally yummy cheeseburger is either logically necessary or else incoherent. But in the case of cheeseburgers, it's easy to argue that it's incoherent. Of all the possible people in all the possible worlds, surely we can find two with incompatible tastes. We could imagine one guy who likes cheeseburgers better the more cheese they have, and another guy who likes his cheeseburgers with the lightest hint of cheddar possible. No one cheeseburger could satisfy both of them. Hence we reject the premise, saying the concept is incoherent.

But it doesn't exactly work the same way with God. People have tried through the centuries to say the three omni-s result in an incoherent being, and the modern consensus is that they've failed. It is possible that such a being is coherent.

Is such a maximally great being coherent or impossible? Many folks throughout history have said impossible, and Plantinga has no quarrel with them (well, he does, but not here or at least not much). And some theologans, Plantinga included, have said the idea is coherent. You might think that, for example, if you found some sort of first cause argument to be persuasive.

Personally, I have no opinion on the matter. I have not heard an argument either way that I find persuasive (though I do think it's theologically unnecessary and practically unlikely). In any case, it's not the sort of thing I care for -- I prefer evidence to armchair philosophy.

That kind of brings me back to why I recommended Plantinga for reading, though. I don't recommend him because he's apologetically persuasive -- indeed, I recommend him because he's not. He talks about whether things are reasonable or unreasonable, whether it is rational to be a theist or irrational to be a naturalist, and what rational even means. He's interesting reading if you think (where this discussion started) that religion as a whole is transparently irrational. I'm not trying to convert you; I wouldn't recommend an apologist, as that's an ideological attack. I recommend Plantinga because I think you'll find him interesting and alien.