A conlang should be good for something, have a reason to live. If described just by portmanteuas: homelang, manifestolang/ideoculturelang, easylang, despressionlang, deaflang.
This is a home language. A language will have the shortest words and the most grammaticalizations for dealing with its primary domain, in this case–talking at home. Any good home language should have verbal affixes to indicate if an action is at home or away from home. It should have a rich vocabulary for dealing with family, children and the significant other.
This is an opinionated language. A language will have compound words, short (root) words and grammaticalizations with an agenda. No particular effort to be culturally neutral like Lojban or fair (like Esperanto) or alien (like Klingon and Na’vi). This isn’t a mindcontrollang like 1984, this is a language that believes that culture does and must inform the structure and vocabulary of a language and can’t be neutral. A language doesn’t control the minds of its speakers anymore than indoeuropean languages are making it’s speakers think and act like bronze age Pontic pastoralists.
Depression and Cognitive Therapy. The language should have features for the rapid detection of biased thoughts, such as catastrophizing and unrealistic expectations of the future.
Easy to learn to speak, but also easy to read and quick to pronounce. This is hard to achieve. Up to now, everyone has assumed that isolating languages like tokipona and hyper-borrowing languages are necessarily easy (most auxlangs). toki pona is an oligo-isolating language that is way over on the isolating end of the spectrum and way over on the oligo end of the spectrum as well. I think that this has made it easy to produce but hard to read and well as verbose. toki pona does lack in the derivational periphrasis department because it wasn’t part of the original design and the community is still somewhat slow to convert words and patterns into productive derivational patterns.
Oligo-agglutinating. Piling on suffixes and prefixes is easy, phonotactically. Also, if enough of these affixes are derivational, then we can achieve the ease of word coinage, without the verbosity of using whole words and phrases.
Phonetically. It should have an extremely large logophone-space– lots and lots of possible words with a significant distance between individual sounds and words. Minimal phoneme pairs and minimal word pairs are not easy to learn. They may evolve over time, but that can’t be helped. What can be helped is how hard the language is on the day of its birth. There should be a machine implementable algorithm for generating new words
Scripts. It should be hand-signable and written using the 24 letters of the alphabet, without diacritics.