Well, you know
Luigi has a great recovery that can deliver more height/distance than kirby's. Most of that isn't practically used. But you can also have a nearly ungimpable recovery but purposefully aiming a charged green missile ledge sweetspot.
He's also got great priority. His Tornado, while in the air, completely ignores most projectiles, and eats up a ton of attacks, and while on the ground, eats up or clanks against a lot of things. Thanks to it's quick startup and movement speed, it's a very cool tool against a lot of projectiles.
But it isn't just that- I've never done testing to see what works where, but I get clanks on a lot of my moves all the time- upsmash, uptilt, and fsmash included clanking with the likes of snake's tilts and people's smashes.(Like for sure Luigi's uptilt always clanks with Fox's fsmash and luigi's up-angled fsmash clanks with marth's fsmash). More often than not- it seems to come as a shock to my opponents- a lot of these players using a characters better moves getting outprioritzed and then outspammed by the likes of luigi's uptilt. I'm not sure what Luigi's dsmash priority has, as it almost always hits and rarely clanks against something.
Which brings me to what is probably my favorite thing about luigi. Attack speed. He has a lot of fast startup, little ending lag moves. When playing luigi, the first thing I keep in mind is that I never have to trade up starting lag or ending lag and make myself vulnerable at any time. Between multiple fast double aerial combinations that autocancel to a jab or dsmash or grab, I pride myself in being hard to hit while still being very aggressive.
Next to that, Luigi is abnormally good at escaping strings of hits and through whatever mechanics is not a guy to suffer much hitstun. Either it be his floatiness, his fast aerials like nair allowing him to attack out of hitstun so quickly, or simply that he suffers less hit stun than most characters makes him great at avoiding getting hit more than once where others can get hit at least twice. (A good example, almost all of Kirby's combos are completely ineffective against luigi where they at least work to a nominal 32% on others- Kirby's fthrow to uair, for example, can usually be chained twice on most characters and once on just about everybody, but not luigi. Same thing with any of his dthrow to uptilt choices- they just don't connect at all on luigi).
Of course on the other end, luigi is quite talented at racking up the damage. His aerials are very friendly to strings of hits, as is his uptilt. Thanks to his flexibility, it seems that he can either follow DI and still hit with a different move very well or can chase through attacks and airdodges just as well. I've tried to pay attention to what I do and it often looks something like this-
Punishing spacing or shielding with a grab to downthrow to uptilt to aerial...
Falling weak hit nair to uptilt to possible more uptilts to aerial...
Dsmash(standing or sliding) at low % to upsmash or uptilt to aerial...
I also sometimes find that I can fair to nair at low %, and at 50%+ I can often still two hit with a weak hit falling nair... I'm sure there is more, but these are just off the top of my head. It's hard to keep track of what all I do when my opponent goes from 0 to 64%.
But are also just starters. I can also start out with an uptilt at nearly any time. IF you hit on the downwards arc of his uair(your back to your opponent, and anytime luigi's foot passes the perfectly vertical point) you can often get two hits in a single shorthop at even 50%+. So as a damage builder, I've done things like a falling weak hit nair to uptilt to uptilt, then shielded as my opponent used his nair out of hitstun thinking to hit me, grabbed, downthrowed, then 2x uair to land(that's an easily autocancelled set of aerials btw) to something like a buffered dash into an upsmash chase or go right into shorthop double bair spacing. It's not all "combo" exactly, but luigi's flexibility in his aerials and moves makes him very, very effective at building up damage despite different moves the opponent take to avoid getting hit.
And last of all, as I've started doing this much more just recently- but want to exploit the incredible defensive power of shields? After performing a short hop double aerial that doesn't hit anything, thanks to luigi's lack of lag and the ability to even avoid landing lag with some aerial combinations, I landed with time to powershielded a marth's fsmash meant to punish me, out of which I simply took two small steps forward and firejumppunched marth out of his fsmash ending lag for a KO at 60%!
Also, KO power. Sub 100% KO % on all characters on Final Destination w/ up-angled fsmash, great variety of other KO moves, etc. etc.
So this long-winded post is just me talking about how everytime I play luigi I'm only motivated to play more luigi.