To go with this approach you're proposing, every power that shouldn't work on incorporeal targets would have to include a caveat "doesn't work on incorporeal" along with a list of other things it doesn't work on. That's cumbersome design, since it's a lot easier just to say it once.
But to say it once, you'll need to account for EVERY thing that it applies to, both now, and in the future.
If you let things decide on a case by case basis you can decide when you design an element whether or not you want it to interact with other elements. You're not locked into one concept.
More to the point, they didn't do it at all. Not top-down, not bottom-up. Wherever that exemption for tripping Casper ought to have been made, it wasn't.
They didn't want it to be? Saying a ghost should be immune to tripping because it can walk through walls I think is largely personal prefference.
I might say that ghosts spend a majority of their time as semi physical beings... Ectoplasm... They can CHOOSE to go insubstantial but it's not the basic state, so they're effected by trips et al.
The point of bottom up though is that I haven't determined that this applies to ALL creature like this one. If I want, later on down the line, I can add a new creature that IS immune to trips, or other types of physical forced movement.
Conversely, the rules for the grab maneuver don't allow for things you and I could do to a person without the benefit of martial powers, much less heroes.
Again because it's the basic spring board for other actions. Whether those actions come from a power, or a use of page 42, grab doesn't have to/ need to account for those actions. It only has to account for the initial grabbing.
Some stuff might require the use of a grab, but grab does not require the use of the other stuff.
I'll try. Characters have a smattering of powers, and they're pretty much stuck with their choices until they can level and respec. If I throw an amorphous/incorporeal foe at them and then put my foot down and declare that it can't be knocked prone or slid around, or tell them that they can't use a sleep spell on a zombie, then I'm not really making them innovate new tactics.
I'm still not sure what you're getting at. Yes- if you take away somethings ability to function then you've taken away that things ability to function. If I take away the ability of a lock to be picked, then I've taken away the rogues ability to pick it...
Anything they do that isn't using one of their powers isn't going to be very effective, and they can't just get new powers on the fly.
Page 42 talks about this.
All I'm basically doing is stepping in and vetoing 4e's implicit promise that the small repertoire of powers at a hero's disposal will be reliable because monsters powers work on everything equally well, whether biped, quadruped, big, small, alive, dead, solid, liquid, gas or ether. That's working against the grain. In 4e, homogeneity is a feature, not a bug.
Yeah... Instead of approaching on a case by case basis you're trying to effect everything all at once, when it's not designed that way.