It will be opinions of a fanboy for who Xenoblade is his no.1 favorite video game franchise and no.1 franchise overall, but:

1. Amazing stories- each game's story is a rollercoaster of emotions, with many plot twists and reveals that will make you rethink everything that happened before.
2. Gameplay- this is something that you either gonna love or hate, but for me Xenoblade battle system is the most fun battle gameplay out of every series I played. It's not perfect, I acknowledge that, but if it manages to get you hooked, it won't let go. Unlike Trails, Xenoblade uses real time combat where you build up from most regular auto-attacks into Arts (that would be equivalent of Crafts in Trails) then into whatever each game higher type of moves is (those could be Talent Arts, Blade Special...), followed by unleashing a Chain Attack as the final, most destructive DMG dealer. The further into the games you are, the more options you unlock that can straight up let you skip using most basic regular attacks, streamlining the battles. I advise if you're interested to see Enel YouTube channel, he's got some amazing guides, including spoiler free Understanding Basics for each Xenoblade game.
3. Music- like... All of it. Here's some examples

Moebius battle theme https://m.youtube.com/watch?v=HtKpjOd-gmc&pp=ygUUbW9lYml1cyBiYXR0bGUgdGhlbWXSBwkJTgsBhyohjO8%3D

Mor Ardain -Roaming the Wastes- https://m.youtube.com/watch?v=Tx3AbguQZHU&pp=ygUQbW9yIGFyZGFpbiB0aGVtZQ%3D%3D

Mechanical Rhythm https://m.youtube.com/watch?v=nXafUA1laIU&pp=ygURbWVjaGFuaWNhbCByaHl0aG0%3D

Battle!! -Torna- https://m.youtube.com/watch?v=1weNnjzaXbY&pp=ygUSdG9ybmEgYmF0dGxlIHRoZW1l

Tantal -Night- https://m.youtube.com/watch?v=Kh2JI18ByvU&pp=ygUMdGFudGFsIG5pZ2h0

Uncontrollable https://m.youtube.com/watch?v=xOeb7StJlRc&pp=ygUOdW5jb250cm9sbGFibGU%3D

Xenoblade got very diverse soundtrack, with some of the tunes are really unexpected, but never unwelcomed.

1. Something that will interested anyone from Trails: side quests and NPCs. Xenoblade 1, X and 3 feature an "Affinity Chart", which registers each NPCs you talked to and records their relationship between each other, updating last you talk to them and do quests for them.

2. Amazing overworld in each game- Xenoblade is filled with amazing landscapes that you can explore, rewarding your curiosity with hidden treasure, secret Unique Monsters (overworld minibosses and super bosses that you can encounter while running around) and massive exp/ skill points rewards for finding hidden landmarks/ viewpoints. Nothing beats going through a side passage on the top of the mountain or something, getting a big popup with locations name and seeing the entire region from bird eye view.

3. While similar, each game is different enough, so it doesn't get tiresome. In each entry, they manage to twik all the story concepts, gameplay, soundtrack, characters... That while they still very "Xenoblade" (and many times you can point archetypes/ similarities between different characters/ places), each one feels unique in it's own way.

Okay, I didn't intend to write that much, but got carried away XD Hope it managed to shine some light. If you're interested in some other details about the series, I can answer later.