Once the limitation is lifted the system has to deal with non-monotonic reasoning (http://en.wikipedia.org/wiki/Non-monotonic_logic) and that leads to multiple inheritance problem (http://en.wikipedia.org/wiki/Multiple_inheritance). Unfortunately, the multiple inheritance problem has, so far, only NP-hard solutions (http://ijcai.org/Past%20Proceedings/IJCAI-89-VOL-2/PDF/047.p...). [EDIT: think of a command "eggs are in a fridge"; to find eggs in a fridge you need to know that they are inside a container; you need to know its shape and how to open it; if your software knows that and inherits this knowledge each time when asked for eggs, then it will brake when the eggs are not in the box.]
We are in need of software that deals with large networks of human knowledge. Then we can take SHRDLU to the next level. Otherwise we are stuck with Will Smith movie (http://www.imdb.com/title/tt0343818/).
[EDIT2: there are many theoretical "suggestions" but very little software or practical internet applications]
2. One big problem when interacting with systems like this I suppose would be that you have to know what is supported by its grammar in order to use it. The best solution I can see to that problem would be to add some sort of smart as-you-type suggestion feature. Making it smart enough would require some research of its own though but probably well worth it.
edit: Somewhat related is a tool for working with grammar for computer applications (parsing and translating between abstract and concrete and between different concrete representations): Grammatical Framework (http://www.grammaticalframework.org/)
Page 290 of the dissertation  discusses ambiguity in words and phrases. The Winograd Schema challenge as defined by Hector Levesque poses questions like "The ball could not fit through the hole because 'it' was too big. What was too big?"
These questions are obvious to humans, but difficult for computer programs to answer, because "it" may refer to multiple objects. If SHRDLU gets confused, it may ask for clarifications from the human, or make its best guess and assume that "it" refers to the green cube rather than the blue pyramid.
 - http://hci.stanford.edu/winograd/shrdlu/AITR-235.pdf
and an online version of part of the system http://www.grammaticalframework.org/~peter/shrdlite/shrdlite...
In short, you've replaced a link to an excellent overview that has a link to more material, with a less useful link. I know we have our disagreements, but this just seems out and out wrong. As I've said before, I find this immensely frustrating.
Please follow the HN guidelines and email us when you have a complaint about moderation. Procedural nitpicking in the threads makes them worse.