1) Small lexicon. Incompletely described languages also have small lexicons. Klingon falls into this category. The lexicon can grow. Esperanto on day 365 was a small language. Something like a century later, it is a large lexicon language.
2) Closed lexicons. All (?) languages exhibit the feature where some classes of words are closed, e.g prepositions in English– you can’t make up your own. Proper nouns in English need only follow the phonotactic rules, make ‘em up all day. If a lexicon is small and closed, then there are still new lexemes, but the will be made of recognizable parts. It’s sort of like, you can’t use new ingredients, but if you make a new recipe, you have to show the recipe. The recipe could still be incoherent.
3) Small distance to your native tongue. This is what really makes language easy. A condialect would be the easiest. The maligned re-lex is small in the sense that really you just need a lexicon and the rules for (possibly mechanically) mapping grammar from one language to the other.
4) Small phonetic inventory. Doesn’t make it especially easy though, cf Hawaiian with the long words with repeated vowels.
5) Small syntax. Regularization reduced size twice- irregular morphology can be looked at as lexical syntax (a new word say for each form of a certain tense), or as a complex set of rules with exceptions and exceptions to the exceptions. However, one of the magic things about syntax is that a small number of rules can in the right hands make a massive maximal sentence with enough complexity to be hard to read and sometimes one more rule would make certain areas of complexity go away. This is essentially the story of the evolution of modern computer programming languages.
6) Small number of speakers. Just for completeness.
7) The speakers themselves are small. Okay, now I’m just being silly.