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.
Base Breaker: You either think he is a tragic figure or a selfish mass murderer. You either love him for being a villain that almost always has the last laugh or you call him a Villain Sue etc. He certainly is one of Marvel's most debated villains.
Counterpart Comparison: Frequently compared to Darkseid. His true counterpart was originally Metron, but Roy Thomas told Jim Starlin that if he was going to steal from the New Gods mythos, he may as well steal from the most interesting one, so he is a mixed Expy of the two characters. Starlin later created Mongul for DC comics, as a second-generation homage of his own character Thanos. And then he created Synnar as a third generation homage of that.
Creator's Pet: In issues written by Jim Starlin that primarily feature Thanos, he's always written so that he would almost never lose for real against anyone. It's either his own subconscious desire to lose or it was a clone. This tends to annoy fans of other characters or even fans of Thanos who felt it was getting too much.
Epileptic Trees: There have been several made of the fact that his chin looks similar to those of Skrulls. In an alternate universe it has been even revealed that his mother was one in disguise.
This actually has an in-universe explanation. The godlike aliens The Celestials went to various different planets and created on them two different strains of races, the ugly shapeshifting Deviants and The Beautiful ElitePhysical God race called the Eternals (humans are a separate but related species predating both; mutants are an offshoot of seperate Celestial experiments on humans). The Skrulls are descendants of alien Deviants while Thanos is a descendant of the Earth Eternals who was born with a mutation that makes him resemble the Deviants. Since his design is based on the unrelated DC character Darkseid, this is a marvellous case of Canon Welding.
Foe Yay/Ho Yay: One might often get this impression when he interacts with Adam Warlock.
Jerkass Woobie: Much more Jerkass than Woobie, but his hopeless relationship with Death is nevertheless tragic.
Moral Event Horizon: Cross this for the first time when he committed genocide on multiple planetary populations for sport, slept with the corpses of some of his female victims, murdered all of his mistresses and offspring, and slaughtered the paradise he was born in, including his own mother.
Rooting for the Empire: He is one of the biggest mass-murderers in the Marvel Universe, yet Jim Starlin and many fans are incredibly enamoured with and defensive of the character.
Ugly Cute: Early in his childhood in the Thanos Rising arc.
Villain Sue: Kept getting more and more invincible and rationalised for under Jim Starlin's pen until he peaked in Marvel Universe: The End, wherein he bonded with the Heart of the Universe, a power source greater than the Infinity Gauntlet. (However, according to editor Tom Brevoort, the story is out of continuity.) And even then, when he's writing the issues, the only way Thanos can lose is if he allows the heroes to win via self-defeatism, and even if he loses it's no biggie, because it was probably just a clone anyway. In fact, the story settings bend so much for him due to favouritism from Starlin that he enters massive Black Hole Sue territory.
Wangst: In contract to the Jerkass Woobie listing above, a lot of people who otherwise enjoy Thanos hate when writers try to play him as sympathetic. The character can be a badass villain, but he never does anything the least bit redeeming, so writers' attempts to make you feel sorry for him because he sabotages his own plans and can't get Death to fall in love with him fall pretty flat.