"The art looked good and people liked it" - me after 99% of show openings.
That sentence may seem simple, but dig a little deeper and it encapsulates the wide ranging emotions faced when reflecting on an opening.
The art looked good - This is what we aim for. If the art looked bad, I think it's safe to say that the show, even if well attended, was not particularly successful. If the art looked OK, then clearly the artist is not entirely happy with the result no matter the response. Some art is not pretty, but that doesn't mean they want the installation to look poorly done. Looking good is more about the quality of space and install than it is about the art being pretty or attractive.
People liked it - again, at least for me, this is a major goal in making art. I don't need people to like everything I do and I'm not trying to be a people pleaser, plus you always want a deeper connection than just "that's cool"...but on the baseline, viewers "liking" your art is a good thing. Whether that's through aesthetics, the experience, finding deeper connection, or just getting to know the artist, if a viewer walks away from a show with a positive view of you, the artist, and the art, that's generally a successful showing.
What's not said - "The show sold out"..."Had a great turnout"..."scheduled some studio visits"...these would definitely be part of the report if they happened. No artist wants to leave a show and simply say "I didn't sell anything" as if that's the only thing that matters...but pretty much every struggling artist I know does think about sales and, when they don't come, question the ultimate value of the art and show (not to mention the effort put in to create it).
Turnout is the same way. I've never gone into a show thinking "if I get X number of people I'm happy" but if you only have 10 over a few hours, you feel the dread creep in. That said, if those people engage in conversation, take their time, talk to you about the art, and/or buy something, 10 can feel like a thousand. On the flip side, if a bus full of drunkards stumbles in and out within 20 minutes or your art is the sideshow to some other event, having a hundred people there doesn't really feel any better. It's quality over quantity, at least to some extent.
Well, I had fun! - There isn't a concrete answer as to what makes a successful show. Different artists have different expectations, goals, and realities, and what may be an uplifting opportunity for one could be a disappointing realization for another. I think its important to focus on the good ("The art looked good and people liked it") but it's also essential to realize things that disappoint you, if only so you can work on addressing them, work toward achieving them, or form more realistic expectations and goals in the future.
It's easy to get down when you work on art for months at a time, spend hundreds if not thousands of dollars and hours working on a project, only to have low turnout and minimal sales, but it's important not to ignore those feelings as well. I leave almost every show I have with mixed feelings to some degree and it's been a life-long challenge to address those in a productive way, but finding "success" in art is about understanding your goals and working to find a happy medium between "the measurables", such as sales, and "the intangibles", like a worthwhile conversation.