What are the defining traits of a Euro-centric conlang?

Question

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).

Answer 1

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:

1. Most of the "core" European languages possess it.
2. Nearby languages (Celtic, Turkic, etc.) lack it.
3. 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.
4. 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.

Answer 2

Expanding on Sparksbet's answer, additional features, from the conlanging point of view as listed by Mark Rosenfelder in Advanced Language Construction (pp. 30-31), that tend to pile up on top of the SAE elements. While many of these are not particularly rare at all cross-linguistically, the overall combination (especially combined with actual SAE features), shines a brighter light on the "Europeanness" of a language:

^{(italics are my additions)}

• A phonology that explores little beyond European languages (usually German, French or Spanish with a few sounds swapped or added)
  • The "standard fantasy phonology" is English consonants + /x/ and (usually) Spanish vowels. Rosenfelder notes in passing elsewhere that English's vowel system is unusual, and most budding conlangers seem aware of that.
• Six distinct pronouns with gendered ones for third person only and usually with object forms
• Nouns paradigm with only singular/plural forms, and possibly case (usually 5-8)
• Adjectives are either invariable or decline like nouns
• Verbs conjugating by person (a single one, too) and number, with only three basic tenses (past/present/future) plus maybe a conditional or irrealis/subjunctive.
• Modality involves auxiliary verbs
• SVO word order + nominative/accusative alignment
• Questions and negatives are formed with some sort of particles
• The number system is decimal and the kinship system is eskimo
  • Even SAE languages have significant non decimal elements: dozens and grosses in English, vigesimal elements in French, and Danish...

Answer 3

Besides the traits of Standard Average European given in Sparksbet's answer, another defining feature is the phonology and basis of the lexicon.

Eurocentric conlangs draw their phonology and their words heavily from well-known (and sometimes less well known) European languages. Depending on the preference of the authors, the words are based on Latin or modern Romance, English, Greek, or Slavonic languages. They also inherit a lot of European phonological quirks, e.g., in the selection of allowed consonant clusters both word-initially and inside words.

Features that are absent or rare in European languages (like retroflex consonants, tone, initial use of /ŋ/) are absent from Eurocentric conlangs.