There are some constructed languages that are extremely precise, yet extremely difficult (Ithkuil), some that are easy to learn, but a bit too flexible (Esperanto), and tons in between. What criteria would a constructed language have to meet to gain widespread use and to be easily usable by the general population?
The "goodness" depends on the purpose. There are several purposes. Some are meant to be used by people in a similar way to how natural languages are used, and meant to be used by several people, like Esperanto. Others are to give flavor to an artwork; a book (Lapine), a movie (Na'vi), a tv-series (Dothraki), a franchise (Klingon). Being used to communicate is then not one of the primary goals. Then again some are experiments: "Can a language with X be mastered by humans? How will this feature Y change when used? How will this feature Z affect its users?". Some are meant to be artworks in and of themselves. Some just happened and later give birth to conventional works of art (Quenya). Some are meant to be secret, used only be the creator or a small group. Some are made to communicate with the divine (not so uncommon in Wicca IIRC). Some are political statements, IMO (Láadan). Some are an attempt to fight depression (Toki Pona). Some are made for role-playing purposes (Tsolyani), as part of the world-building. Some are made to learn/discover the principles of linguistics. Some are made as a source of examples to teach linguistics. Some are made to serve as a contrast to languages already known, in order to better understand the mechanisms of those. There are probably fewer purposes than there are conlangs but I don't think there are that many fewer.
Some have a goal of passing as a natural language. Then it becomes possible to use the normal linguistic arsenal of tools to analyze them and discuss them. Those who do not have this as one of their goals is probably a bad fit for this site.
Some are meant to be a complete system usable for communication, others are not. As with naturalism, the former can lead to fruitful discussions, the latter are a lot less interesting, at least to me.
It would need to be politically expedient to learn it. Like it is now with English and as it was for Greek. The process could be sped up by the simplicity of learning the language, yet people will learn any language that they NEED to learn. This is more a political issue than linguistic, at least in my opinion.
To my opinion, as long as a language is not insanely difficult, constructed languages will only be of a very limited range of complexity, since communication and languages are by themselves complex. This, and the fact that children seem to be able to perfectly well learn even the most puzzingly complex language, leads me to the assumption that all questions as to whether a language will be gaining widespread usage will not be dealing with intrinsic properties of that language, but much more with external factors.
One can make a case that modern Hebrew is in a way a constructed language (even if it is not constructed from nothing) and it did gain widespread adoption; and for Esperanto there's nowadays native speakers as well. However, in most of the cases I believe people simply don't see the reason for learning a constructed language. Linguae francae are generally acquired from culturally dominant and prestigeous groups, but constructed languages simply have no such group of speakers that would inspire others to take their lead.
In any way, what is certain, is that once a language - constructed or not - is spoken routinely by a sizeable number of speakers, it is inevitably going to change.
- Ease of learning, reading, and pronunciation. Therefore there can't be consonant clusters (imagine Russian), or sounds not in most languages (ex. English's þ, ð, and w, the distinction in its two l sounds, or its many vowels), and the alphabet/syllabary should look distinct even if the person hasn't learnt it yet (ex. katakana, runes, hobo symbols, or most of the roman letters - not Arabic, Braille or kanji). This is not just so that the language doesn't feel impossible, but also to help hard-of-seeing people and dyslexic people who recognize letters slightly more based on "shape" (it's hard to explain, but telling the difference between o and 0, or n and h in bad handwriting, can be very hard for example).
Of course, it needs to be able to be written on mobile phones and computers with very little effort.
This also means there needs to be very little grammatical exceptions (if any), and all of the most common words are very short, ideally one syllable (ex. "is, eat, need"). Otherwise the language is cumbersome. There also needs to have a flexibility like Esperanto, wherein you can say either "I go" or "I am going" depending on construction, but the usage is restricted to individual's tastes. The point is that, people like things that are customizable. They want to be able to play with the language and feel like it's their own, so it can't be so rigid. (This is also why videogames such as The SIMs and Don't Starve get really popular - it's basically free choice on how you want to play the game.)
There also can't be tones or pitch to change meaning, as this is largely seen as difficult. Basically, look at Japanese and Finnish.
- Ease of word order. While a language like English has very fixed word order and relatively simple grammar, it not only makes it difficult for people from drastically different languages, but the problem comes that if you're not allowed to have a different word order and convey the same meaning then you're going to lose support of everyone in favour of a "more logical" language (ex. you should be able to be correct as long as the correct meaning is understandable through common sense). Likewise, the language has to be tolerant of mistakes, as people generally have a fear of being wrong and the culture that eventually arises from the language should be friendly towards learners (not "i'm better than you at it, it's difficult so i'm smart") in order to not discourage people in learning.
The same is true for vocabulary. It needs to ALL make sense through common sense. For example, a "water cooker" needs to be able to mean a "tea kettle" and be correct.
It needs to be learnt in a fast amount of time, and be very, VERY easy but still not lacking things. This is especially true since you will need to capture large amounts of monolingual speakers, for example most English-speakers. The average person from the USA doesn't even feel like they have enough time in the day to cook dinner "from scratch" (I've even heard complaints about how "it takes such a long time for water to boil for my instant ramen"), but at the same time English speakers pride themselves on the small nuances that come in word pairs like "handrail, banister, balustrade", and often believe that other languages don't have them. So the constructed language has to heavily cater to both of these aspects.
There needs to be proof that the language works and that people speak it. Meaning, before you even publish the book that teaches it, set up a forum for it, translate material and write original material in it, have some audio recordings, etc. People already say things like "you're learning Icelandic? WHY?" because they believe it's useless and there's nothing worthwhile in it, so you have to combat this very strongly starting from the very beginning.
The vocabulary has to be usable in daily life. You need to be able to write blog posts or software in the language, for example. At the same time there needs to be technical words too, like for textbooks. Otherwise people either see it as useless or as not "sophisticated enough".
- The last and most important step is that you need lots of money for advertising. I don't mean ads as in the traditional way, I mean to create products that use the language - for it to get big, it needs to be in cartoons, tv shows, on product packaging, etc. People need to just go to their favourite website and notice the name of the language on the sidebar, for whatever reason. It needs to show up when Google searching.
You can get online news and bloggers to write about it, you can ask indie companies about putting some words of it on their CD cases and organic soy milk, etc. Basically it has to somehow reach the mass public without them in any way actually looking for it.