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.
Anticlimax Boss: Loewe used to be this, but after Falcom released a patch to upgrade him, he's sometimes considered to be harder than even the last boss.
Awesome Music: Metric tons of it. There is no bad music in Trails, only tracks that are marginally less awesome than others.
"Silver Will" is more than worth mentioning, since it isn't an overstatement to say that this music may be even more popular than the game itself. Hell, it's even listed as the Best RPG Battle theme on 2ch and Youtube! And then there's the vocal version, which takes it even further.
"Hoshi no Arika" or "The Whereabouts of the Stars", ranked first on several "Best Game Theme Song Ever" lists in Japan. Give it a gander.
Doing It for the Art: How else do you explain XSEED Games's willingness to localize a trilogy of games with such ginormous amounts of text, which no other localizer showed signs of having plans to work on?
Ensemble Darkhorse: Renne. She was popular enough to have one of the Drama CDs focus on her, which wasn't planned originally.
Naturally, Renne's popularity led to her being playable in Alternative Saga, too.
Epileptic Trees: One's worthy of noting outside the WMG page, with the speculation that this game may take place in the future of the world Ys is set in. Of course, this crops up with other Falcom games and is usually scoffed at... but then Alternative Saga came out and the shaking trees caught fire. The Word of God hasn't explicitly clamped down on this or confirmed it... yet. Another popular option is that it takes place in the far, far future of the same world as the Gagharv trilogy.
The idea that the Kiseki world is the future of Ys seems exclusive to the English-speaking (and non-Japanese-reading) fandom; the game itself says the characters were drawn from different worlds (the setting is also largely derived from Xanadu Next but nobody seems to think that it's the same world as the Ys or Kiseki games) and it's pretty clear from the lack of reaction to Chester and Loewe and Lloyd not recognizing Renne or the Brights in Zero no Kiseki that the events of the game didn't 'really' happen. The connections to Gagharv are much more solid, mostly centered around the Doll Knight novels from Cagesong of the Ocean.
Then there's the painting of Gagharv found in The Third's prologue. One could guess whether the painting is just from the painter's imagination or not.
Even Better Sequel: While the backstory and setting of FC is sweeping in scope, the combat is fun and the characters likable, FC ultimately plays a fair number of tropes relatively straight at least up until its very last act and even comes across as a little cliche at times, and really serves as more of a setting-establishment piece. It was SC that completely blew the lid off of player expectations (and a whole mess of tropes) across the board and cemented Trails' place in history and the Japanese gaming zeitgeist. (Given what initial reactions to SC's announcement were, there's a level on which this is intensely amusing.)
This could be applied for this series to the previous "Legend of Heroes" series, too.
Game Breaker: Agate in SC, Alan in 3rd. In SC, Agate has a skill that allows him to potentially spam S-crafts indefinitely. For Alan, he has a combination of awesome stats, a surprisingly good Orbment setup, and super-low cooldown on his Crafts so he can get lots of turns. Not to mention that his second S-craft, Ouka Zangestu, is one of the most powerful moves in the game.
In Alternative Saga, Loewe. His Arc Drain has the ability to drain the enemies SP, if it hits 3 enemies it would refill the skill's AP cost and add an extra for it so the skill pays for it's own cost. Not to mention his Kienzan is not only fast but also has the power that rivals an Extra Skill.
God-Mode Sue: Cassius is OP, both as an NPC and when he is playable. When he appears, you can very much assume that the heroes' behinds have been saved from whatever impending doom might befall them, no exception.
Magnificent Bastard: Cassius. A rare case where such a character is on the protagonist's side. Mostly.
On /vg/, spamming "TITS SC WHEN" is this. Another variation is "TITS ON STEAM WHEN".
Also, GameFAQs seems to be fond of making fun of people who question Estelle's behavior towards Olivier.
Moment Of Awesome: Loewe vs Joshua. Multiple people have called the fight, without irony, even more epic than the entire climax sequence of Advent Children. Made even more impressive by the fact that it's all done WITH SPRITES, and it still manages to be completely awesome.
Villains may they be, you've gotta admit the scene when the Legions storm the Grancell Castle is completely awesome; it's four Badasses versus one Badass Army and our dear villains proves to be the more badass force.
Both of the Storming the Castle events in the final chapter of FC. Made even more awesome in that both times the heroes are able to pull it off with a group of less than 20 people!
Nightmare Fuel: Man, by the end of the main plot a lot of characters have generous helpings of this in their backstories. The younger Ouroboros members like Joshua and Renne have it particularly bad. And then there's what Weissman does during the game, and what ends up happening to him... Needless to say, despite being ultimately optimistic this game gets really, really dark at times.
Weissmann's S-Craft, Another Dimension. Full stop.
No Export for You: This is being fixed at last courtesy of XSEED Games, although it still took nearly seven years for the first chapter to make the jump across the sea, and for a long time fans had resigned themselves to never seeing a release. (The fate of the PC version remains a question-mark.) The delay is mostly attributed to Falcom's past rocky relations with other localizers/publishers in America and the sheer, staggering size of the scripts for each game. The second chapter had to ship on two UMDs for the PSP release, after all.
Portmanteau Series Nickname: With the English version, the game's initials are "TiTS". Whether this is a bad thing or not is best left to the viewer.
Sequel Displacement: The trilogy-cum-franchise has proven to be so popular that it's basically supplanted its parent Legend Of Heroes franchise; the following two games are Zero no Kiseki and Ao no Kiseki, both of which still technically keep the "Legend of Heroes" franchise title but practically as a footnote, and don't even bother with numbering any longer. And the next one, aka Nayuta no Kiseki drops "The Legend of Heroes" altogether, but that's because it's a Spin-Off. Sen no Kiseki restores "The Legend of Heroes" because it's a main entry, though.
Ship Mates: Joshua/Estelle shippers tend to support Agate/Tita and Olivier/Schera as well.
As sequels popped out, they add these ships in their list: Zane/Kirika, Loewe/Karin, Lechter/Kloe and Kevin/Ries.
That One Boss: Loewe in SC, Cassius and the Black Knight in TC. Loewe takes the cake though, he is so hard the game actually let you choose to proceed the game without defeating him!
Renne also tends to make people pull their hair out in SC, particularly the last fight with her when she's accompanied by Pater Mater. It's bad enough that she can use her reaper-esque S-Crafts to one shot your entire party if you're positioned badly. When she's accompanied by a mountain of steel and HP that can punt you across the arena and revive her when she's knocked out? The suffering never ends.
Grimoire!Kevin and his cannons from The Third. For starters, he likes to spam Grail Sphere EVERY TURN. And the cannons...OHMAIGAWD THE CANNONS. Unless you bring Kloe for her +50% Def S-Craft bonus, you won't survive against their 3-hit cannonballs.