Well, I love ISFPs, as I am married to one! <3
Some key differences I've noticed: ISFPs are more in the moment and in their bodies, feeling their senses, than INFPs. I can get lost in my head thinking. I can ask him, "What are you thinking?" and he kind of gets this deer-in-the-headlights look and explains he was literally not thinking about anything and is now trying to think of something to think. I can't imagine sitting there without a thought in one's head, blankly, just experiencing life (sounds rather zen, actually), but he is very good at it. ISFPs, if they are like my husband, don't get drawn into long, weird, meta-focused musings about the nature of the world, universe, life, non-life, etc. He's extremely intelligent, started college at 15, achieved a lot, etc, but he doesn't waste thoughts. He does enjoy every sensory experience he can, though. I love that because being with him helps me appreciate experiences more.
It isn't my experience that INFPs have trouble applying themselves. It may be a common thing but I've not experienced it. Maybe I'm a little bit of an ISFP because I have a few passions/skills I like to hone, finished a novel, etc. My ISFP husband doesn't really fit the mold, either, because he isn't really the creator type. He does not write or create visual art or really have creative output of any kind.
I experience higher highs and lower lows than him, on average, emotionally.
He is less sensitive to negative physical sensations than I am like hunger etc,.
I am more empathetic; he asks me often how to word things tactfully. I don't know if his tact "problem" is due to personality or just other factors.
He absolutely CRAVES travel, while I find travel to be a great way to experience the world but I don't feel a "lack" if I stay in one place for a long time.
Similarities: We're both procrastinators. Ugh. Haha. We both tend to be a little bit cluttery, but since we finished college the house tends to stay a lot more presentable as we both have more time to pick up after ourselves. We both have about the same level of tolerance for mess, so that's good. I'm a little more absentminded than him, more likely to forget to throw the dryer lint away (which bothers him) but I'm more likely to get down to business and clean something. (Also I am more likely to be resentful of having to clean that thing if he said he was going to clean it and then didn't get around to it for four days or a week or whatever.)
If I am upset about something, I tend to let him know, whether or not there is anything we can do about it. He's usually pretty straightforward with me, too. However, one time I found out SIX MONTHS LATER from him that something I had done or failed to do had hurt his feelings. I was distraught; why didn't he tell me sooner so I could fix it? He said he did not want to distress me and it could not really be fixed so there was "no point" in telling me. Still, though, it was good to finally be able to apologize once I was aware...!
We both love animals (cats particularly).
We both struggle with melancholy and depression.
We both draw deep enjoyment from nature and the world around us, but in different ways. We both deeply appreciate beauty in the world.
We both hate injustice and believe in standing up for what is right and fighting oppression.
We both like cuddling.
We both need our alone time, but handily, time "together" doesn't sap us if we are both doing our own introverted things separately in the same room.
We both care deeply about people and desire close relationships.
Another difference here: In life, it is common for once-close friendships to drift apart over time. I, the INFP, react to this with laissez-faire acceptance. I feel like we will always have a common thread and that in the future if we ever want to talk again at any time we can pick right back up, but if they need to move on with life it's all good. I feel like if I reach out once or twice and don't get much back, it would be forcing it to ask for more. For my ISFP husband, when such friendships fade it is *extremely* painful for him. It's just agonizing. He feels it as such a loss. Along with being empathetic, I try to comfort him by saying that we have been close to *so* many people in our lives that it is physically impossible to keep up with all of them forever; there aren't enough hours in the day. He says he knows but it's still horribly painful anyway.