Pushing Power: My main issue with swain is one that is rarely mentioned, but in my opinion quite cripling, his pushing power. Most mages max there pushing ability first (or don't have pushing issues for whatever reason,) while swain would prefer to max his (W) last, and at best can max it second. This puts him into a freeze lane + abuse catagory, which is not the most effective, since your lane opponent may walk down to dragon, or gank another lane, then come back to his tower to soak up the wave you slowly built up while painstakingly lasthitting with 50 damage autos. His ult sucks for pushing ultil he has rank 2-3 and lots of AP.
Immobile Drain Talk: Since swain has basically no mobility and just has to run into a team and be a man, this makes him very suspecitble to being burst down. He needs MR, Armor, HP, and Mana to teamfight effectively. He opens ROA (a slow starter) then build Frozen Heart/Hourglass + Abyssal/Spirit VIsage, and IMO isn't particularly strong until he has both resistances. Other top lane champions require much less in the way of itemization (ie Rumble) or have the mobility/range/hourglass rush to allow for effective teamfighting earlier. Swain needs time to deal his damage, most mages don't.
Suspectible to ganking in top.
Worthless if behind, (due to close range damage, lack of mobility, requirement of tankiness)
Don't get me wrong, I like swain and I enjoy playing him. He makes melee champions cry and flaunt his manliness in his enemies faces. His builds just come together slowly, and maxing W or Q second always leaves some kind of hole in his power, though W max is good enough. His W basically relies on his targets to be CC'd or retarded to land. I do think teleport surplanting ignite in top lane, in combination with his mana restoral on kill or assist do help his viability quite a bit. He's just not much of a playmaking champion, takes time to ramp up, and is very mediocre if not ahead, which is why he sees little to no competitive play.
Adding some innate resistance to his ultimate would help him out considerably, perhaps resistance scaling with ability power? Increased damage against minions would help his ult a lot, though his healing vs minions would have to be reduced accordingly. Kind of an ugly solution but it would help a lot.