Story/Plot + art. Most of the time it's a combination of both for me, and most of my favouritest series have incredibly great art and even better stories (Changge Xing/Song of the Long March, Vinland Saga for example). Sometimes the series could have mediocre art and a great story (e.g. Shingeki no Kyoujin) and I'd be hooked in till the end. Even if the art isn't great but the story-telling is impressive, I'll follow it! Some series have such jarringly remarkable art that I'd dive head-first into it without so much as a glance to its plot (e.g. Daisuke Igarashi's Majo, which also has great stories. What an impressive short series. It's one of my forever faves and this man is an artistic genius); at other times, I might not be too fond of the art/it's not to my taste but follow for the exemplary story (URASAWA NAOKIIIIII I love him so, thus have developed a fondness for his style - he's defo not a bad artist; he's great - it's just not to my taste, much)
p.s. when reading scanlations, finished/unfinished series definitely come into consideration, along with translation and editing quality.