Lucario's only real weakness is the lack of a safe approach against characters with good pokes, like Samus or Link's aerial grapples.
People have been placing a lot of things as weaknesses that shouldn't really be. Slower than average smashes aren't that big a deal, and dsmash out of shield is fast enough to hit when it counts anyway
(out of shield or after powershielding a punishable attack).
KOs aren't as difficult to get as everyone is saying they are. Dairs, even after being spammed an entire stock, account for most kills I get with Lucario, usually off the stage. The move's properties make it perfect for well-zoned ledge pressure, as well as dropping quite far off most stages for low % KOs on opponents trying to recover from below (even in Brawl, bad DI happens). Aura Sphere is obviously also good for KOs, especially out of a powershield. Outside of those two, dsmash isn't as hard to pull off as people are saying, uairs at high % also work, especially having not been spammed, bairs are strong although somewhat hard to connect, and ftilts can either kill at high % or lead to dair gimps in lower %.
Lucario has an amazing utilt, which is a beast out of powershields or normal shields, and often you'll be chaining 2-3 of these with uairs or fairs. I don't know why I bring this up, no one is questioning Lucario's ability to string together attacks, but I love this move a lot.
The point is, his main weakness is the lack of a safe approach. His smashes are fine, his kill moves are fine (though admittedly not earthshaking), and his recovery, for the love of Odin, is FINE. Are people forgetting he can curve the angle, thus being able to sweetspot in AT LEAST two different ways in any given situation? I shouldn't even have to mention that he can wall cling from his up B.
Other disadvantages, which I wouldn't exactly call weaknesses, are a subpar tech chase game and what appears to be a very short ranged grab.
EDIT: Oh, and he isn't light at all. He's middle of the road, and can live much longer than most.