In my opinion, Aaliyah was an average, or slightly above average artist overall.
She was an OK vocalist - but she was no Aretha Franklin or Whitney Houston - now those two are GREAT singers.
She had a good mix of R&B and hip-hop - but she was no Mary J Blige or even Ashanti (both of whose debut albums, What's the 411? and Ashanti, were almost flawless blends of contemporary R&B with contemporary hip-hop)
Stylistically, she never found an edge. She was nothing unique and lacked the individual style of say, TLC or Janet.
She was not a songwriter - her debut album was entirely written and produced by R Kelly, her second album by Timbaland, DarkChild, Missy Elliot and a couple other folks. Her third, self-titled album was entirely produced and written by Timbaland, Static Major, Rapture and J-Dub. Even by this time, she hadn't even as much as co-written any song, so she was effectively a front for other people's talents.
Comparisons to Alicia Keys are ridiculous. I don't even like Alicia, but she's a stronger vocalist as well as a songwriter and competent pianist.
It's worth noting that Aaliyah got her break into the industry through her uncle being married to legendary Motown singer Gladys Knight. It was Gladys and her then husband (Aaliyah's uncle) who took her out on the road at a young age, got her an agent and ultimately a record deal.
However, when she went to college herself, she studied drama. I would suggest that this was her real passion, and had she lived, her focus would have been less on the music and more on the acting, where she actually stood out and gave memorable, believable performances.
