major technical: no aggregates

There is no ability to perform aggregates, ie, computations on a
set of results.  Example: find authors who have written 2 or more books.
Solution: SELECT ?author WHERE { ?author v:wrote ?book1 .
?author v:wrote ?book2 . FILTER (?book1 != ?book2) }
Now try to find authors who have written 50 or more books.
This will require 50 book variables and 50*49/2 filter predicates.
It would be easier if there were a way to count the number of books
that an author has and then filter on the count.   Other commonly used
aggregates are sum, average, maximum, minimum.  I don't know of any
workaround at all for computing sum or average in SPARQL.  There is
a workaround for maximum and minimum, though it is messy.
Example: find the vendor with the lowest price for a widget.
SELECT ?vendor
WHERE { ?vendor v:sells ?product .
        ?product a w:idget .
        ?product v:price ?price .
        ?vendor2 v:sells ?product2 .
        ?product2 a w:idget .
        ?product2 v:price ?price2 .
        FILTER (?price2 > ?price) }

Fred Zemke