Reading the January 2010 rules update I came across the following:

QUOTE:
Dark Depths
The Marit Lage token that this card creates is inherently legendary, is inherently black, and inherently has flying, because all of those are characteristics. If you copy the token with Sakashima the Impostor, for example, Sakashima will gain those characteristics too. However, being indestructible isn't a characteristic; it's just something true about the token. That Sakashima will not be indestructible. This is wildly counterintuitive, especially since the Marit Lage tokens we printed to go along with Dark Depths say "This creature is indestructible" right on them! The determination is that this functionality loophole is an unintended oversight, and it's being corrected.

New wording
Dark Depths enters the battlefield with ten ice counters on it.
{o3}: Remove an ice counter from Dark Depths.
When Dark Depths has no ice counters on it, sacrifice it. If you do, put a legendary 20/20 black Avatar creature token with flying and "This creature is indestructible" named Marit Lage onto the battlefield.

Now to me the new wording is no less counter-intuitive than the original, and I don't really understand what makes something a characteristic. Here's the definition:

QUOTE:
109.3. An object’s characteristics are name, mana cost, color, card type, subtype, supertype, expansion symbol, rules text, abilities, power, toughness, loyalty, hand modifier, and life modifier. Objects can have some or all of these characteristics. Any other information about an object isn’t a characteristic. For example, characteristics don’t include whether a permanent is tapped, a spell’s target, an object’s owner or controller, what an Aura enchants, and so on.

I have highlighted abilities, because characteristics include abilities. The rules give an example

QUOTE:
"Example: “[This creature] can’t block” is an ability.

Can anyone explain to me why "This creature is indestructible" is not an ability and therefore copyable?

its something to do with indestructability itself. its something a creature is, rather than an ability the creature has. Why this is though, I have no clue.

I think (I'm sure a judge will be along soon) that dark depths with the old wording says that the Marit Lage token is indestructible, without actually giving it the ability "Indestructibility".
So, although the token was indestructible, if you were to create a copy, the copy would not be indesrructible as this was just something granted to the token.

As it says in your quote, this makes no sense (especially given that the token has indestructible printed on it), so the new wording explicitly gives the token the ability so that it can be copied.

I suppose its something that never really came up before, as i) the card was never played, and ii) its legendary, so its rare that you can copy without both copies then dying.

jedi_jon wrote:

QUOTE:
I think (I'm sure a judge will be along soon) that dark depths with the old wording says that the Marit Lage token is indestructible, without actually giving it the ability "Indestructibility".
So, although the token was indestructible, if you were to create a copy, the copy would not be indesrructible as this was just something granted to the token.

As it says in your quote, this makes no sense (especially given that the token has indestructible printed on it), so the new wording explicitly gives the token the ability so that it can be copied.

I suppose its something that never really came up before, as i) the card was never played, and ii) its legendary, so its rare that you can copy without both copies then dying.

Hmm... there's no such keyword as indestructibility.

The example from the rules implies that "This creature is indestructible" is an ability (non keyworded), and the rules tell us that abilties are copyable characteristics.
The rules change is saying it is not a characteristic.

I just need to understand why it's not a characteristic.

Hmmm . . .

QUOTE:
Dark Depths
The Marit Lage token that this card creates is inherently legendary, is inherently black, and inherently has flying, because all of those are characteristics. If you copy the token with Sakashima the Impostor, for example, Sakashima will gain those characteristics too. However, being indestructible isn't a characteristic; it's just something true about the token. That Sakashima will not be indestructible. This is wildly counterintuitive, especially since the Marit Lage tokens we printed to go along with Dark Depths say "This creature is indestructible" right on them! The determination is that this functionality loophole is an unintended oversight, and it's being corrected.

From the rules:

QUOTE:

112.11. An effect that sets an object’s characteristic, or simply states a quality of that object, is different from an ability granted by an effect. When an object “gains” or “has” an ability, that ability can be removed by another effect. If an effect defines a characteristic of the object (“[permanent] is [characteristic value]”), it’s not granting an ability. (See rule 604.3.) Similarly, if an effect states a quality of that object (“[permanent]” is indestructible” or “[creature] is unblockable,” for example), it’s neither granting an ability nor setting a characteristic. (See rules 700.4 and 700.5.)
Example: Muraganda Petroglyphs reads, “Creatures with no abilities get +2/+2.” A Runeclaw Bear (a creature with no abilities) enchanted by an Aura that says “Enchanted creature has flying” would not get +2/+2. A Runeclaw Bear enchanted by an Aura that says “Enchanted creature is red” or “Enchanted creature is indestructible” would get +2/+2.

and

QUOTE:

700.4. If a permanent is indestructible, rules and effects can’t destroy it. (See rule 701.6, “Destroy.”) Such permanents are not destroyed by lethal damage, and they ignore the lethal-damage state-based action (see rule 704.5g). Rules or effects may cause an indestructible permanent to be sacrificed, put into a graveyard, or exiled.

700.4a Although the text “[This permanent] is indestructible” is an ability, actually being indestructible is neither an ability nor a characteristic. It’s just something that’s true about a permanent.

So I believe (though I now confess to being more confused than when I started), that the token was granted indestructibility, but this is not a characteristic. However, by having the text "This creature is indestructible" it has an ability that can now be copied.

Not sure if this helps or not. . . (or even if its correct).

jedi_jon wrote:

QUOTE:
If an effect defines a characteristic of the object (“[permanent] is [characteristic value]”), it’s not granting an ability. (See rule 604.3.) Similarly, if an effect states a quality of that object (“[permanent]” is indestructible” or “[creature] is unblockable,” for example), it’s neither granting an ability nor setting a characteristic. (See rules 700.4 and 700.5.)

OK, that seems to cover it.

This all boils down to a creature token with the characteristics black, 20/20 and the ability flying, and has been granted the quality 'is indestructible' by the effect of Dark Depths. And you can't copy a quality.

I wonder how many of us would do this correctly under tournament conditions.