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.
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.
Jerkass Woobie: Much more Jerkass than Woobie, but his hopeless relationship with Death is nevertheless tragic.
Moral Event Horizon: Readers thought that he crossed the "line of evil" 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, where he bonded with the Heart of the Universe, a power source greater than the Infinity Gauntlet. 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.