My memory is marked by the titles of two papers by Branko Grünbaum:
Branko Grünbaum. `Are your polyhedra the same as my polyhedra?' Discrete and comput. Geom.: the Goodman-Pollack Festschrift, ed. B. Aronov et al, Springer (2003), pp. 461-488.
Branko Grünbaum. `The Bilinski Dodecahedron and Assorted Parallelohedra, Zonohedra, Monohedra, Isozonohedra, and Otherhedra'. The Mathematical Intelligencer (2010). DOI: 10.1007/s00283-010-9138-7.
The first title is easy for me to recall whenever I need to refer to the paper. The second title sounds fancy (though the article itself is not) and, more importantly, is unpronounceable by me, therefore I have put some stretch of mental effort into memorising it.
As to the original question---What makes the title of a paper memorable?---, personally, when I look for things to read, my attention tends to be captured by titles that are short and sweet, for instance, Jean-Pierre Serre's Trees, Ken Brown's Buildings. These monographs/papers usually turn out to be the authorative treaties of the topics, with material unforgettable for one working in the field.