I've seen majority of Aho's games last season since he plays in Kärpät (the team I support) so I can probably provide some insights of his abilities.
First of all Aho is a really smart and defensively responsible player for player who is only 17yo. Kärpät had quite lot of offensive talent in their team last year including promising youngsters like Jesse Puljujärvi (possible TOP5 pick in 2016 Draft), Saku Mäenalainen (lead goal scoring in WJC 2014). Also Kärpät had pretty stacked offence in general so when Aho cracked the lineup last season it alone can be considered as an achievement of some sort. From those three youngsters (Aho, Puljujärvi and Mäenlainen), Aho was the one who got the most playing time. And his play elevated one step during playoffs which is a pretty positive sign. Aho basically displaced former Sharks prospect and NHLer Lukas Kaspar from the second line and Kaspar had had pretty good seasons in KHL. Although Kaspar play sucked in playoffs....
Aho lacks a bit of size but his not a midget and may still even gain few inches. Despite of his size his playing style is pretty gritty and his not afraid going in to board plays against much larger defenders. His skating is pretty good but should probably work his top level speed but overall decent skater with good balance. Has really good hands and hockey IQ which makes it possible to play him in various roles from defensive third line to scoring line role. Not the best shooter and anyway clearly a pass-first short of guy. He probably has a potential to be 2nd line playmaking winger with great defensive instinct but I would think that Aho is a guy who will make steady progress instead of huge development jumps so it could mean that it could take 3-5 years before he is NHL ready.
Next year Kärpät doesn't have that stacked offence so there should be a place for Aho in a scoring line with power play minutes. So next year is great time to see whether he really has NHL potential in him because at least he will be getting enough ice time to prove it.
I, too, watched most of his games last season, and there are a couple of points I disagree with.
-He's better than just a "decent skater". Maybe not McDavid-level good, but he's still very agile and skating is one of his strengths.
-"Anyway clearly a pass-first short of guy." No, just no. When I think of "pass-first" players, I think of players like Joe Thornton or Henrik Sedin, not Sebastian Aho. Aho, in my opinion, is more of a goal scorer, and I expect that his goal to assist ratio will be pretty balanced down the road.