Tl;dr: Esperanto possesses about 8/12 SAE features
Let's go through these features one by one. This is going to be pretty long, sorry for that. For reference, I'm using the famous Haspelmath paper as reference for which features define the SAE sprachbund, as I did in my earlier post.
1. Definite and indefinite articles - ✗
Esperanto defies SAE norms out the gate here, as it only has a definite article.
libro - "a book"
la libro - "the book"
This is actually quite bizarre, as those SAE languages that do not fit this criterion (such as Slavic languages, which heavily influenced Esperanto) have no articles at all. However, some neighboring non-SAE languages do have only a definite article, so this isn't too bizarre cross-linguistically.
2. Relative clauses with relative pronouns - ✓
Haspelmath characterizes this way of forming relative clauses, unique to SAE languages, by the following criteria:
- The relative clause is post-nominal
- There is an inflecting relative pronoun
- This pronoun introduces the relative clause
- The relative pronoun functions as a resumptive, signalling the head's role in the relative clause.
Esperanto relative clauses fit all of these criteria.
Tiu blua balono, kion mi estis tenanta, knalis
"That blue balloon, which I was holding, popped."
Here, the relative clause kion mi estis tenanta follows the nominal head balono. It is introduced by an inflecting relative pronoun kion, which is inflected for the accusative case to mark the balloon's role in the clause (that it is the object of estas tenanta. Pretty much prototypical SAE here.
3. 'Have'-perfect - ✗
Many European languages used a form of "to have" plus a passive participle to mark what we can loosely call the perfect. The languages that possess this construction have grammaticalized it to different degrees, but according to Haspelmath, "what is important here is that they all must have had basically the same meaning when they were first created."
Esperanto does not have a 'have'-perfect. Instead, Esperanto emulates Slavic and Finno-Ugric languages in using "to be" + an active participle -- a form Haspelmath explicitly contrasts with the 'have'-perfect in his article.
Mi estas vidinta la viron.
mi est-as vid-i-nta la vir-on
I be-PRS see-PERF-ACT.PTCP the man-ACC
"I have seen the man."
Unsurprising given Zamenhof's language background, but not SAE.
4. Nominative experiencers - ✓
With "verbs of sensation, emotion, cognition, and perception," there are two main strategies for expressing the experiencer -- either the experiencer is treated like an agent and marked as a nominative subject (e.g., "I like it"), or it is treated like a patient or goal and the stimulus is marked as a nominative subject (e.g., "It pleases me", "It is pleasing to me"). English pretty much exclusively uses nominative experiencers, but this isn't strictly binary across languages -- many languages, including quite a few SAE languages, will have some verbs that do the former and some that do the latter. However, most SAE languages have a very strong tendency to prefer nominative experiencers.
Esperanto primarily uses nominative experiencers.
- Mi vidis fantomon. - "I saw a ghost."
- Mi timas la mallumon. - "I fear the darkness."
- Mi flaras florojn. - "I smell flowers."
Esperanto does possess at least one verb that bucks the trend, plaĉi.
Ĉokolado plaĉas al mi
"I like chocolate" (lit., "Chocolate is pleasing to me.")
However, even this construction is gradually becoming less used over time as speakers favor the nominative experiencer verb ŝati instead.
Mi ŝatas ĉokoladon.
"I like chocolate."
Therefore, I think it's safe to say that Esperanto is pretty SAE in this regard.
5. Participial passive - ✓
To quote Haspelmath: "Standard Average European languages typically have a canonical passive construction formed with a passive participle plus an intransitive copula-like verb ("be", "become", or the like). In this passive the original direct object becomes the subject and the original subject may be omitted, but it may also be expressed as an adverbial agent phrase."
Esperanto passives behave pretty much exactly in this way. Unsurprising, since none of the language families from which it draws its inspiration differ from the SAE trend here either.
La knabo estas batita (de sia fratino).
la knab-o est-as bat-i-ta de si-a frat-in-o
the boy-NOM be-PRS hit-PERF-PASS.PTCP of REFL-POSS brother-FEM-NOM
"The boy is hit (by his sister)."
Granted, these passive participle constructions in Esperanto can often be avoided using anticausitive morphology or free word order in contexts where this type of passive construction would be unavoidable in, say, English. But this construction exists and is certainly frequently used enough to be considered a nicely SAE feature of Esperanto.
6. Anticausative prominence - ?
Esperanto possesses both a causative affix -ig and an anticausative affix -iĝ, both of which are used very frequently.
fand-i fand-iĝ-i brul-i brul-ig-i
melt-INF melt-ANTIC-INF burn-INF burn-CAUS-INF
"to melt (tr.)" "to melt (intr.)" "to burn (intr.)" "to burn (tr.)"
To say for certain that Esperanto has a preference for one over the other, some proper analysis of the language's vocabulary and usage would have to be done. In my limited experience, I'd say that they're about equally common in Esperanto, but it's really not possible to say without a lot more research into Esperanto verbs and their usage.
7. Dative external possessors - ✓
As @jknappen has pointed out, Esperanto does allow for dative external possessors
Mi lavas al mi la manoj.
"I wash my hands." (literally, "I wash to me the hands")
I can't personally speak for how common they are, but the only requirement for this criterion is that they exist.
8. Negative pronouns and lack of verbal negation - ✓
This is another easy one, as this SAE feature is explicitly spelled out in the basic 16 rules of Esperanto:
When another NEGATIVE word is present, the word NE [English no, not] is omitted.
Mi ne venis Neniu venis
I not come No.one comes
9. Particles in comparative constructions - ✓
There are quite a few ways of marking comparatives, but particle comparatives like the English "than" are pretty overwhelmingly SAE. Esperanto uses a particle comparative, as its ol is pretty much identical in function to English "that".
10. Relative-based equative constructions - ✓
Esperanto's equative construction tiel ... kiel is actually more transparently based on relative clauses than the English construction, as it uses synchronic relative correlatives -- and even the English "as ... as" construction is considered relative-based by Haspelmath. Another transparently SAE feature.
11. Subject person affixes as strict agreement markers - ✗
It's far more common, cross-linguistically, for languages with person-marking to be pro-drop. However, several SAE languages (including French, German, and English) inflect the verb to agree with the person/number of the subject without allowing the subject to be dropped.
While Esperanto is not pro-drop, to my knowledge, Esperanto does not inflect verbs to agree with the person or number of the subject, and so it avoids this SAE feature.
12. Intensifier-reflexive differentiation - ✓
European languages verb frequently distinguish between intensifiers and reflexive pronouns, but using one form for both appears to be more common worldwide. English actually averts this, as it uses its reflexive in this intensifier function (e.g., "He did it himself").
Esperanto's intensifier particle, mem, is distinct from its reflexive pronoun si.
Li batis sin.
"He hit himself."
Li mem batis ĝin.
"He hit it himself."
Thus, Esperanto is SAE in this regard.