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. Averted: Hongki aside the band has also two more good-great singers (Jaejin and Seunghyun, who are the subvocalists) and one solid one (Minhwan). 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 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 a few years ago after Hongki and Jaejin, yet as of now he's 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.
Possibly rivaled by Minhwan who's not even officially a subvocalist in the group yet has participated in 3 musicals, 2 of them where he was a lead.
The songs where everyone gets to sing is this: "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.
While the weakest singer, even Jonghun is somewhat improving - Beautiful World with Seunghyun is evidence of this.
It is now tradition for Minhwan to sing Hongki's long high note in I Confess performances.
They now regularly write and compose their own songs, and pretty good ones at that. Here's a regularly updated list of all the songs they've worked on. Definitely on their way to becoming real artists, if they're not one already.
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.
Jaejin's rare bass solos. Notable ones are "TV Radio" and a briefer one in "Live Like a Musical".
There's actually parts in Lovesick where he's the only one playing and he plays some nice melody lines - they're unfortunately not very noticeable due to Hongki's singing and/or the orchestra/piano.
Seunghyun's guitar solos, especially since he doesn't get to do them often because Jonghun's mostly in charge of them.
YMMV on the song itself, however Seunghyun's acoustic guitar solos in "I Wish" definitely count (and they're probably the only cool thing in the song).
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 performances. 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.