To the question "What is most important", I don't think we have formalized any particular strategy for priorities. What I can come up with is the following strawman, to initiate the discussion so that we can iterate on it.
1) Infinite loop. The first priority is always to ensure that no input to the parser/tokenizer, no matter how crazy it is, can cause the parser/tokenizer to get trapped in an Infinite loop. It is hard to guarantee that of course, nevertheless when such a situation is discovered, it needs some attention right away.
2) Negative impacts to the JavaScript run-time, this can be in form of excessive memory consumption, exhausting the stack, or any other situation where the JavaScript engine running the parser/tokenizer can not operate well. A reasonable input to the parser/tokenizer must not trigger such a scenario.
3) Incorrect output. The parser/tokenizer must always produce the expected syntax tree/token list (following the ESTree specification). If the output deviates from the expectation, e.g. when the type of a node is wrong, this can indicate a serious flaw in the parsing/tokenizing process. When this problem is not fixed, it causes a compatibility problem between Esprima and other tools which expect ESTree conformance.
4) Valid program mistakenly flagged as invalid. Not fixing this can hamper the use of Esprima in a tool (e.g. code coverage, auto-complete, etc) since that tool may not be able function properly when encountering the corner cases.
5) Invalid program mistakenly treated as valid. This is less damaging that the above. However, it can cause a wrong expectation to an Esprima-based tool, especially when it focuses on correctness (e.g. static analysis, etc).
6) Speed, or rather, the lack thereof.
Of course, there is also the aspect of implementing new syntax features found in the latest specification. For instance, currently ES2017 is the upcoming standard and Esprima 4.0 is intended to tackle that (
https://github.com/jquery/esprima/projects/1).
I hope that clarifies the situation!
Best regards,