A few nits.

Christophe Grandsire scripsit:

> (grammatical prefixes
> are extremely rare, derivative prefixes are a bit more common)

The only grammatical prefixes that come immediately to mind are German ge-
(the past participial marker) and older English a- (lexicalized "on" before
present participles, now mostly lost).

> - case and number marked on nouns, using a nominative-accusative system.

Ergativity is common in Indic, which is the largest single branch.

> When the neuter is present and there are cases, the neuter
> is *always* specific in having an identical form in the nominative and the
> accusative.

Yes, this is a *very* IE trait.

> (although Slavic languages
> like Russian seem to have a pretty thorough system where each and every tense
> is doubled according to aspect),

Better I think to say that each *verb* is doubled according to aspect
in derivational but unpredictable ways; sometimes pfv is derived from impfv,
sometimes the other way.

> Person marks tend
> to disappear on the verb, obliging people to use personal pronouns), although
> many IE languages seem to do very well without it :)) (look at Russian for
> instance :)) ).

I think this is overgeneralizing from Germanic and French (which toked a heavy
hit of Germanic in the past).  Most IE languages are pro-drop, as are most
languages worldwide, I think.

> relative subclauses
> always follow the noun

There is a world-wide tendency to postpose heavy modifiers.

> (and are introduced with a relative pronoun referring
> back to the noun the subclause completes, giving its function in the
> subclause).

Not always.  In Germanic, that-clauses are dominant.  (Funny how Germanic's
so often the outlier.)

> In questions, reordering the sentence is common. VSO becomes the
> predominant word order, and when a question word is present it is normally
> always first, whatever its function in the sentence.

I think these things are universal in IE.

> And if your conlang's most features are found among the list I gave, you can
> safely say that it's an IE or IE-based conlang.

I don't think that any set of grammatical features can make a language IE
without a vocabulary derivable from PIE.

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