Long netted an eighth-minute opener but then missed a couple of glorious opportunities either side of half-time as Martin O'Neill suffered his first defeat since taking over as Republic coach.
The Hull striker - keen to impress in Robbie Keane's absence - scored with a tidy chip after pouncing on Branislav Ivanovic's poor back-pass, but two further one-on-ones were wasted.
Serbia stopper saved the first on 30 minutes before Long pulled an attempted chip wide of the post shortly after Serbia had equalised through James McCarthy's own goal. Filip Dordevic netted the winner for Serbia on the hour mark.
Long accepted that to some observers, the poor finishes may have undone a lot of his good work at the Aviva Stadium: "I did a lot of things right and caused a few problems but, sadly, my game will be remembered for the two misses.
"Nine out of 10 times, you would take those chances in training but unfortunately they didn't go in."
Long agreed that he scored with his most difficult opportunity: "On the second chance, the keeper has gambled - you could say he read me like a book, I suppose.
"The third one, I've just dragged it.
"Luckily, though, it wasn't a qualifier. The gaffer tried a few things and hopefully I've given him something to think about.
The two opportunities Long wasted came after fine penetrative passes from Wes Hoolahan, who was named Man of the Match.
"It was nice to play with Wes, said Long. It's a striker's dream when you have someone like him playing behind you."
Despite the result, Long was keen to emphasise the pluses for O'Neill's side.
"Serbia offered a good test. They like to get the ball down and play football and that is what we are going to be up against in the qualifiers.
"There are a lot of positives."