What I very well might rant about is people being oblivious to or not even factoring in other peoples' needs/personalities - so ranting about people who have a lack of interpersonal awareness of how their own actions impact other people - or in extreme cases, people who simply don't care how their own self/behaviors impact others.
Edit: Now, tying what I just said to the OP and what the 'istp' was saying, the bulk of her venting was tied to how the isfp's actions and behaviors were impacting not only the lives of the children (others) but also the lives/situation of the church and others. Focus obviously wasn't on how the isfp was doing as a person, emotionally, and her background and issues and feelings and what she's going through, but how all of her choices are impacting everyone else - not accounting for or knowing what other peoples' needs might be, but rather going with what works for her and how she does things. Although, as a result of how the isfp wasn't accounting for others, in the mind of the istp, the isfp therefore got lowered in her estimation and was therefore judged negatively as a result.
I also relate very much to the Te slant of lack of competency/ability to follow through.
Relate to Ti too.