-Character designs: Seriously, the costumes are really cool, and unlike most fighters, they actually change, which is nice. (though some look similar to past costumes, e.g. SC4 Cassy 1P compared to SC2 Cassy 2P.)
-Mechanics: Sure, it's simple, but simple games are often the most popular (Look at MvC compared to say, Virtua Fighter.)
-Music: The music can be both comedic (Midnight Mystique), epic (Endless Warfare), or just about anything, really.
-Style Diversity: OK, ignoring Rock/Asta, Cassy/Sophie, and the 3 mimics and 3 S&S in SC5, the characters are diverse both in motif and in style. Seriously, sure TTT2 may have 50+ characters, but half of them are similar in some way, in terms of gameplay.