Hongki's singing. While technique-wise he is average in kpop note note that the singing standard amongst idols isn't good to begin with and majority of kpop main vocalists are average so this really isn't an insult, his singing - which relies on emotion, musicality, interpretation, voice - is what makes him a highly regarded singer, and what got him so much attention right from debut. While he doesn't do a perfect job himself, he's clearly the only one who can pull off the band's ballads.
The difficulty was hilarious lampshaded in reality show Cheongdamdong 111 where a group of trainees performed for them and CN Blue. They revealed to have chosen CN Blue's song over FT Island - only because they found the latter's song too hard to sing thus switched to the former. Watch Hongki and Yonghwa's disappointed vs smug looks switch around.
You'd think in a band only the main vocalist can sing well while everyone else can't sing a tune. In FTI however, Hongki aside, the band has also two more good-great singers (Jaejin and Seunghyun, who are the subvocalists) and one solid one (Minhwan who isn't even an official subvocalist). Only Jonghun's singing is very limiting and could be labelled as bad but even he sounds alright while in his comfort zone.
Would you expect 4 out of 5 members of a rock band to be worthy of participating in musicals? Most would say no. Yet, while Hongki is a given, Seunghyun, Jaejin and Minhwan have surprised and impressed many people with their singing through these.
Seunghyun is quickly becoming the resident musical actor of the group: he only joined his first one after Hongki and Jaejin, yet in a span of 2 years he already participated in 4 sets of shows within 3 musicals, more often than everyone else. Comes to show how passionate he is about them and how quickly recognised he's become.
Minhwan possibly even rivals Seunghyun, as he's only the drummer yet participated in 3 musicals, 2 of them where he was a lead, in a span of less than 2 years.
The songs where everyone gets to sing is this as they all sound decent if not good and could give a few kpop boy bands a run for their money in terms of overall singing skills: "The One note back with Wonbin, who himself is a decent singer and was capable of going solo", "I Knew From First Sight", "It's U", "Treasure", "I Confess", "Someday", "I Hope You'll Be My Lover".
Jaejin and Seunghyun's singing have come a long way. Both were made to be subvocalists to make up for Wonbin and while they've always had pleasant voices they were quite shaky at first. Years of practice, experience, musical work and so forth have not only made them better and more stable live but much more confident too. This has lead to increase of lines for both of them.
It has become tradition for Minhwan to sing Hongki's long high note in "I Confess" performances, and he does so fantastically.
After a few years of music mostly strictly written/composed by others, they regularly write and compose their own songs, and pretty good ones at that. Definitely on their way to becoming real artists, if they're not one already. The notable composers are Jonghun and Jaejin who have written the most out of everyone, however note that everyone has written songs for the group.
A fully self-composed Korean mini-album with a song they made (Memory, written and composed by Hongki in this case) finally being made a title track for their Korean comeback was this. Shame that said awesomeness was tainted when FNC decided to clash its promotions with their concerts thus the barely-existing promo on TV shows only lasted two weeks.
Whenever the members get to do really cool solos or flashy techniques with their instruments in their songs.
Jonghun's guitar solos which often make the highlight of the songs, such as "I Hope", "Severely" and "Tornado".
Jaejin's rare bass solos. Notable ones are "TV Radio" and a briefer one in "Live Like a Musical".
Seunghyun's guitar solos, especially since he doesn't get to do them often because Jonghun's mostly in charge of them. Examples include "Moonlight Angel", "Morning Coffee" and the acoustic guitar solos in "I Wish".
While he unfortunately gets no solos in their songs, Minhwan still showcases the extent of his drumming skills in many songs, such as "beautiful", "BE FREE" and "Tornado".
Them being the first foreign male group to top the Oricon Album Daily Charts with their debut album in 42 years with "Five Treasure Island" is definitely this. They also remained at the top for 2 consecutive weeks.
Jonghun playing both the acoustic guitar and electric guitar in older "Hello Hello" lives (not so much nowadays). He'd have his electric one on him while playing the acoustic one on a stand up til 2/3 of the way into the song - then he would leave it there and immediately play his electric guitar without trouble.
He does a similar thing for "Memory" lives, except he'll change between electric guitar and keyboard - more than once.
Hongki's Immortal Song 2 performances, as the music show pits noteworty singers up against each other to showcase their vocals - and of course Hongki's were always amazing in terms of singing and stage presence. Up to Eleven when the others are there to back him up with their instruments and also prove their skills.
Jonghun's "Somewhere Over the Rainbow" guitar solo is one of his best guitar performances if not the best.
Also an older performance of his guitar solo version of Canon.
Once when Hongki posted a photo of him and his new scooter, many Korean netizens bashed him for apparently violating road rules and not having a licence plate. Hongki responded saying that he was in the clear since he had just bought it and parked it in front of the shop, hence out of harm. Most idols would stop at that; however Hongki also called out certain netizens for always blindly bashing idols and actually said, "Hey you keyboard warriors, if you're going to hate on me, at least know the facts."