Subselects lack functionality

Kuba Ober (winnie(at)hoth(dot)amu(dot)edu(dot)pl) reports a bug with a severity of 3
The lower the number the more severe it is.

Short Description
Subselects lack functionality

Long Description
1.
Subselects don't allow one to use tuple set operators like UNION, INTERSECT, EXCEPT. It forces one to select the results into a temp table.
2.
It would be gr8 if single column subselects would allow themselves to be treated the same way as lists do as right-side arguments of IN operator.
This would allow e.g. much more powerful DELETE statement, like
DELETE FROM table WHERE id IN (SELECT ...)
3.
Subselects cannot be used as arguments to aggregation functions - I presume this is very weird and not expected at all, but I had such an idea.

I don't know if it is expected behaviour (standards?), this should be explained in the docs (or am I searching in the wrong place?)

Sample Code
select (select id from table1 except select table1.id where expression) as id;

(assuming that subselect would result just one tuple)

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