Passive: A pain to proc pre-6 or during the early game in general. However it comes into its own during teamfights when you can pull of a full ability combo more easily and can be quite damaging if pulled off right.
Q: A typical poke attack, however the side bolts are a bit finicky land on anything bar jungle creeps outside of heavily clumped team fights (And even then most teams don’t tend to stand in perfect lines perpendicular to your attacks!). One idea would be to allow each bolt to deflect +/- 10 degrees or so to target the nearest enemy champion or monster. Either that or allow a third q press to cause the side bolts to explode and do damage.
W: I feel this is a bit too easy to spam this one at the moment. One idea would be to only start the timer to generating another stack on your W after the cooldown for casting a stack of it has elapsed and/or if you get down to 0 stacks of W and need to generate more, then reset the timer so you have to wait another full timer before you get another one. This would encourage smarter casting of the W, as opposed to spamming it, not dissimilar to shooting at the enemy cowboy’s feet and watching him dance like in spaghetti westerns.
Another change I feel would benefit the W would be to make it cast like Rumble’s Equalizer AKA Drag and click in the direction you want it cast. This in my opinion, combined with the above would make it more rewarding to land a successful W. Opening a rift to the void shouldn’t be taken lightly!
E: A knock-up? Yasuo will love this guy. The radius is a bit on the small side and could probably be sidestepped by anyone with boots or a ms over 345, but it is very useful for displacing enemies. No real complaints here.
R: His party piece, and probably my new favourite ult! It’s incredibly satisfying to disintegrate people with it and it moves very fluidly. My only criticism is for something player controlled it’s a bit on the short side, and having very little windup to prepare means you have to factor in player reaction time, so you have to really be on point. Lengthening the windup on it would give a bit more of a chance to run away for the enemy team, give a bit more time to move your cursor to the right place, and quite frankly it’d look cooler.
That's my two cents on the subject. Overall he's fun to play and he's been designed well, with just a few small things i'd iron out. His lack of escape stops him from being too much of a powerhouse and makes the player think about positioning and escape routes. His AP ratios seem in line and seem well suited to mpen. On first impressions I would probably play him against champions with a lack of escape to capitalise on the slows and knockups he has.