These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Moe: Consider the genre. There's a lot of this to go around with the girls, but Ein in particular is adored, especially in Japan. "Ekukko" is the favored subject of a lot of fanboys' art pages...
Most Annoying Sound: Go through the Mireno Cemetery puzzle involving the ankhs and colored crystal alters that manipulate the water level in the subterranean lake. Now go through it without wanting to stab Ein in the face every time he chirps, "I can hear the sound of water!" Such a challenge is even tougher than the boss battle.
Ditto for the obligatory underground water ankh puzzle.
Tear Jerker: Ledah's death at the end of Chapter 6. Also watching Malice and the girl who likes Ein the most being sacrificed by Hector. Although the heroine gets better.
That One Boss: The Second Accursed, Archangel. Her Overdrive will most likely kill Ein in a single hit if he doesn't have enough status or protection and will cause good damage to the rest of the party and some of most annoying status effects. May the game have mercy on you if that affects the entire party. Oh, and she's all but immune to the Holy element, which is on most of your strongest weapons. Have fun!
Even with maximum possible levels, The Sage and The Fool in chapter 4 can cause a lot of players headaches.
Hector can easily become one if you rely on one element or he picks one he is immune to most your party's attacks. Kill him as fast as you can to avoid this problem.
Weirdly, the first boss you have to face, the phoenix Aghart. Not because it's particularly difficult or risky - it's impossible to die at this point in the game - except that Ein probably WILL get knocked out since you haven't been able to raise his skills yet. And Ledah only uses fire, Aghart is a fire beastie... this one's gonna go on for a while, before the story even gets started.
That One Sidequest: Oh, so you want the only available non-drop Rosier in the game with multiple uses? S-rank a Bomb Kid and never let any of them use their Overskills on you. (read: Suffer.)
Mention skipping stones to anybody who has played Riviera to completion, especially if they tried to play a high-score run. To quote one Let's Player:
Didja Redo: Hey, Heinrich Himmler called. He wants to know if you're offering cruelty lessons.