Next to one of the busiest avenues of modern Athens, the arch (or gate) of Hadrian was built during the reign of Roman Emperor Hadrian, probably in 130 or 131 C.E.
Unlike the typical Roman arches, which tend to be bulky and sturdy (like this), this one is slender and light. It is if the architect went out of his way to make this arch different from the others, or perhaps to make it fit the other monuments of the city.
Standing under it, one can see the Acropolis on one side, and the Temple of Zeus on the other. In this photo, taken from the West, the columns of the latter are just visible under the trees behind the arch.
Aristotle, Greek tourist guide 
Interesting and quite a bit lighter than Titus’ arch as you mention. But Titus’ arch is a victory arch. Here in Jerusalem there are two remains of gates from Aelia Capitalina, the Roman city built by Hadrian, one below the present day Ottoman gate, Damascus gate and the other at the entrance of what was the agora, today called Ecce Homo on the Via Dolorosa. Both gates are triple gates, a large Roman arch with a smaller arch on either side.
Good point, SB, Hadrian’s gate in Athens is not a triumphal arch (celebrating military victory). However, the stylistic difference with other Roman arches does not lie in its function, but on the message whoever commissioned it wanted to convey.
If it were Hadrian, the choice of the elegant classicist style fits beautifully with the political persona he wanted to project: cultured and refined, and a lover of classical Greece.
If it were the Greeks (conquered and part of the Roman Empire at the time) the arch was probably meant to convey something like “Hey, you may have won, but in matters cultural we’re still better: look how beautiful our arch is, compared to those bulky affairs of yours.”
Of course I may be reading too much into it and the style of the arch could simply have been chosen to fit with its surroundings, especially the newly built temple of Zeus nearby.
Who knows? Whatever the reason, its commissioners made an excellent choice: the arch is a beautiful thing to behold.