"A quick heads up here - There is some fairly experimental stuff on Jayce that's going into the outgoing PBE build. You guys won't have access to this for a little while as PBE builds timing and pushes, while regular - are always on a 'as soon as we can' basis.  You may see some craziness in the next few days from Data Scrapers but this content is really ridiculously rough.  1. Jayce should not ignore resistances.
Yes, I know no one levels up 'R' anyway at the moment - but for the moment, let's pretend that Jayce theoretically shouldn't ignore your resistances.  2. Jayce's poke pattern is fairly unhealthy.
Mostly from a fog of war, long range Q+E standpoint. 
3. Jayce's burst shouldn't hyperscale and his damage shouldn't be dependent on optimizing really weird obtuse use cases.
This mostly pertains to the 'W' functionality changes - where people are 'rolling' W charges to string together a 6 shot hyper charge. Also, Hyper Charge probably shouldn't deal 260% damage on a critical strike - or 325% damage on an IE crit strike.  4. No, seriously. A lot of Jayce's power is locked up in really obtuse cases.
A little bit of why Jayce is so powerful in competitive play and not so much in regular play is the sheer amount of edge cases you have to abuse to get mileage out of Jayce. While some could argue this is mechanical mastery of the champion - a lot of it is simply unintuitive fodder that we should clean up so that his actual intuitive combos are powerful.  So what's going on?
We're testing a fairly experimental series of changes designed to shift power away from these three cases and into the rest of his abilities - as well as streamlining some of Jayce's muddier patterns. In general, Jayce is a decent character to have in the game but the vast majority of his power is locked up in extreme abuse cases at the moment - especially with end game super burst scenarios or long range poke scenarios.  I'll have some more specifics as we start to refine what we have - but here is a rough list of the major changes:  Passive Statistics
Shifting around his statistics to favor mana regeneration as Jayce should be more of an active laner focused around spells to achieve victory. The shift in regeneration should also compensate for the removal of Mana on Hit from Static Field.  HP/5 reduced to 5 from 7
HP/5 per Level reduced to 0.5 from 0.7
MP/5 increased to 10 from 7
MP/5 per level increased to 1.0 from 0.7
Jayce may no longer level Transform but his abilities have 6 ranks.  Hextech Capacitor
Moved a portion of his transform passives onto his character passive. Additionally, Hextech Capacitor is weak at character levels 1-3 but stronger at 6 and 9 to smooth out power differences between the fact that he has multi-form abilities and no ultimate.  * Now additionally grants 5 to 130 magic damage for the first attack after transformation, based on character level.  To the Skies!
To the Skies! had some early game spike power and late game spike power. Normalizing this curve such that you don't have to stack a ton of early Attack Damage or a ton of late Penetration in order to utilize this ability effectively for burst.  * Mana Cost fixed at 30 at all ranks from 40/50/60/70/80
* Damage rescaled to 25/75/125/175/225/275 (+0.6 Bonus) physical damage from 20/65/110/155/200 (+1.0 Bonus) physical damage  Static Field
Static Field should encourage Jayce to stay in Hammer form and engage on his opponents for prolonged periods of time while his Cannon Transform is unavailable.
Also experimenting with a W -> R -> W swap combo - though the current gameplay is a little unintuitive. 
* Passive mana gain on-hit removed.
* Damage rescaled to 60/110/160/210/260/310 (+1) from 100/170/240/310/380 (+1)
* Basic Attacks during static field now deal an additional 10/15/20/25/30/35 (+0.2) magic damage on hit. Numbers even more highly experimental and subject to change on this one. * Cooldown rescaled to 13/12/11/10/9/8 from 10  Thundering Blow
Thundering Blow in top lane was often Jayce's easy-mode button. If Jayce was in a bad spot - he could simply Thundering Blow to escape. Aggressively gating the Mana Cost on this ability means that eventually, Jayce's second chances will run out.
However, we wanted to preserve Jayce's engagement pattern if he chooses to be active - hence the To the Skies! Mana changes. 
* Mana cost fixed at 85 from 40/50/60/70/80
* Now has a rank 6 that deals 23% health damage and up to 700 magic damage versus minions.  Transform: Mercury Hammer
* Now grants 5 to 35 Armor and Magic Resistance, based on character level.  Shock Blast
Changes here are mostly to increase the dodgability of Shock Blast, especially from fog of war + acceleration gate circumstances.  The acceleration of gate also favors using E after launching Q - as this will give the opponent the smallest amount of time to react.  * Damage rescaled to 60/105/150/195/240/285 (+1.2 Bonus) from 60/115/170/225/280 (+1.2 Bonus) - Probably needs to be scaled up a tad. * Missile Speed increased to 1450 from 1350
* Missile Width decreased to 50 from 60.
* Missile Area of Effect decreased to 160 from 175
* Accelerated Missile Width remains at 60.
* Accelerated Missile Area of Effect decreased to 225 from 250
* Accelerated Shock Blast initial speed increased to 2550 from 2350
* Accelerated Shock blast now decays down to 1550 during flight  Hyper Charge
Hyper Charge harass becomes more dominating depending on the rank of the spell in a lot of cases. Early ranks of Hyper Charge tend to be less than mana efficient in a lot of ways. These changes are designed to reduce some of obtuse abuse cases of Hyper Charge with IE or priming the skill but to also make Hyper Charge's cost roughly equivalent to its effect.  * Damage reduced to 70/80/90/100/110/120% from 70/85/100/115/130%
* Critical strikes now simply add 100% of your total AD to the damage (this can be further modified by critical strike damage) rather than being dependent on the rank of the skill.
* Cooldown rescaled to 14/12/10/8/6/4 from 14/12/10/8/6
* Cooldown now begins when Hyper Charge is fully consumed
* Mana Cost rescaled to 30/38/46/54/62/70 from 40  Acceleration Gate
Jayce brings Acceleration Gate as his primary team utility. The Jayce player should feel confident that his team contribution shouldn't be necessarily gated by his playstyle or what he is forced to build. The cooldown nerfs in 3.9 should control Gate''s spamminess and uptime - and thus the Mana Cost here seems to be unnecessarily restricting him to Mana intensive builds.  * Mana Cost reduced to 20 from 50
* Has a 6th rank that increases Movement Speed by 55%.  Transform: Mercury Cannon
This proc effect was mostly unnecessary - while it ensured Jayce made it to late game, the combined effect of the proc means that Jayce effectively ignored far too much of his opponent's armor than was healthy.  * No longer has an armor and magic resist reducing proc."