[Impl0x]

There are a few "easy" answers for ease of adoption, some of which I think have been mentioned by other replies. A phonetic alphabet with only one possible sound per letter, the ability to identify a word's part of speech given only the word, and an exception-free verb conjugation scheme are all common approaches for making a language easy to learn. Some constructed languages have a simple rule for determining which syllable in a word gets emphasis.

Also worth considering is making the language as "familiar" as possible to a new learner. You could decide what phonemes your language uses by looking at the most frequently used phonemes in the most popular languages. You might decide to go a level higher and base your language's word for universal concepts (food, day, sleep) based on roots that are common across many languages.

Another way to make a language easy is to reduce the number of words a person needs to know in order to use it in the real world. I'm personally a big fan of Esperanto's word-building system, which uses around 30 affixes (in addition to the suffixes that define each word's part of speech) that can be attached to a root word to modify or enhance its meaning. There's not a limit to how many affixes you can have, the word just has to make sense. Every time you learn a new root word, you get a huge number of new words for free by knowing affixes. A language that has such a system can get by with a reduced number of root words. For example: "laboro" is the word for the word "work" or "job" in Esperanto. By adding only one affix you can arrive at words like "labor-ej-o" (workplace), "labor-ist-o" (a laborer by trade), "labor-ant-o" (a person performing labor), and "labor-ind-o" (a job worth doing).

However, it's not really enough to just make a language easy. I'm glad you brought up virality; drawing people in is almost certainly the more difficult part. Once you have your ideal language, you need to build a culture around it. This is the same thing as when people judge the quality/utility of a programming language by its "ecosystem". Unfortunately this is largely out of the language creator's control. There's just too much work involved for a single person, or even a small group of people. There needs to be an abundance of learning material, lots of translated works and a growing body of original works (across as many genres/media/levels of proficiency as possible), and networks for speakers/learners to communicate and collaborate.