It has a completely regular grammar and allows the creation of a large quantity of words by combining lexical roots and about forty affixes (for example from the radical san- (healthy), it is possible to create words such as: malsana (“sick”), malsanulo (“sick person”), gemalsanuloj (“sick people of both sexes”), malsanulejo (“hospital”), sanigilo (“medicine”), saniĝinto (“person who has recovered”), sanigejo (“curing place”), malsaneto (“little illness”), malsanego (“extreme illness”), malsanegulo (“very sick person”), sanstato (“health state”), sansento (“health feeling”), sanlimo (“health limit”), malsankaŭzanto (“pathogen”), kontraŭmalsanterapio (“therapy against sickness”)…). The main parts of speech (nouns, verbs, adjectives and adverbs) have consistent endings that always allow the recognition of all parts of speech. Its regularity makes it particularly easy to learn, and its streamlined capacity to create new words make it one of the most productive languages, with a potentially unlimited number of words, it is capable of expressing all new ideas or states. For example, it is possible to write a novel about fictional table-shaped Martians and to call them tablo (“table”), tablino (“female table”), tablido (“table offspring”)… We can imagine a man who walks backwards ( inversmarŝanto , “reverse-walker”), a remedy against dogmatism ( maldogmigilo , “undogmatizer”), etc.
Or the textbook that I used back when I tried studying the language (Jon Rømmesmo - Esperanto - Lærebok - Lernolibro):
Du kan - det viser forsøk som er gjort - lære esperanto på ca. 1/6 av den tida du ville bruke på eit nasjonalspråk. Med ei lita ordliste kan du sjølv lage deg eit stort ordforråd ved hjelp av forstavingar og etterstavingar.
Translation: Studies have shown that you can learn Esperanto in a sixth of the time you would use on a national language. With a small list of words you can make yourself a large vocabulary with the help of prefixes and suffixes.
Tell me, what are those, if not invitations to "invent" language. Literally no natural language permit this kind of free-form creation of vocabulary, not even those filled to the brim with prefixes and suffixes. There are always strict rules and conventions that limit what you can do. Not so in Esperanto, if these descriptions are to be believed. And even if these descriptions aren't true, why should I be chastised for believing in them, given that they're written by supposedly authoritative sources? Is it my fault that I took this at face value, and ended up not participating in the larger Esperanto culture, where, presumably, I would've learned otherwise?