title_body: What are the defining traits of a Euro-centric conlang? I know that a Euro-centric conlang is a conlang based mostly on Indo-European grammar, or more generally a conlang that is written from a worldview (conscious or not) that the "normal" language is Indo-European, but what are the traits of a Euro-centric conlang? What are the things that are a dead giveaway that the language is Euro-centric? 
  
  It could be a grammatical curiosity specific to Indo-European, or maybe something like a conlang being based around one non-Indo-European language feature as a kind of "gimmick" (although these are just unfounded speculation, seeing as I don't know the answer).
upvoted_answer: The term "Standard Average European" (SAE) pretty much covers it, and has been around since the 1930s. Haspelmath listed a number of typical "Euroversals" in a portion of the 2001 book Language Typology and Language Universals. These are listed in a more readable-to-laymen way in the wikipedia article on the subject. Haspelmath included as true Europeanisms (that is, features that are part of the SAE Sprachbund) only features that fit the following requirements:
  
  
  Most of the "core" European languages possess it.
  Nearby languages (Celtic, Turkic, etc.) lack it.
  Eastern Indo-European languages (Armenian, Iranian, Indic languages, etc.) lack it -- this prevents us from merely calling it a feature of the Indo-European language family as a whole.
  It is not found in the majority of the world's languages.
  
  
  The Europeanisms identified by Haspelmath that fit this criteria include:
  
  
  Definite and indefinite articles
  Post-nominal relative clauses with resumptive relative pronouns
  The (transitive) perfect formed with "have" plus a past participle
  Experiencers coded as nominative subjects
  Passive formed with a participle plus "to be"
  A preference for anticausatives over causatives.
  Dative external possession
  Negation with negative pronouns and positive verb forms (e.g., "Nobody comes" instead of "Nobody won't come.")
  Particle comparatives
  Relative-clause based equative constructions (e.g., "tan Z como X", "so Z wie X", "as Z as X")
  Subject person marking used strictly for agreement (i.e., the verb is inflected for the person/number of the subject but the subject cannot be dropped)
  Different forms for reflexives vs. intensifiers
  
  
  He also identified several other features that were common among this Sprachbund but were not as well-documented as the above, such as the verb fronting of polar interrogatives (i.e., turning "I have done it" into "Have I done it?" to make a yes/no question), an inflectional marker for the comparative, and several others -- these are also included in the linked articles.