I wrote this:
This means that, as with any distributed application, concurrency is key: we have at least one flow of execution per node running concurrently with all others, and [...]
I was told to change it to "is the key". Is that right?
"Is the key" implies that it is the only, or most important, factor. "Is key" means it's important. Which did you mean?
I think something that is missed here is that using "key" as an adjective (key point, key findings, etc.), although common for quite a while, is still somewhat debated, but using it predicatively (something is key), is only recently in vogue by some, and definitely not standard. So when the initial questioner was told to change "is key" to "is the key", it was to use standard English, perceived variations of meaning notwithstanding. (Since we are on a language usage page, I should point out that Chad should have used "implies" instead of "infers" in his comment.)