Well Riot openly said in the patch nerfing Leona's damage that they thought her burst with Aftershock was too much so that's why they tuned her damage down. So keeping in mind that it's only been like a week since that change came down, and the fact that Leona is still really successful right now (according to champion pick/ban and win rates on public sites like LOL King and OPGG) it's very possible that she specifically is still overtuned and it's also possible that shockwave is overtuned and is the real culprit. Will have to wait to hear more from Riot on this as the season gets closer.
I personally also agree that there is too much focus on early game burst damage and snowball early leads. Not just like early/mid game champs either, snowballing right now is an issue even with scaling champs (i.e. maokai going aftershock instead of grasp or guardian).
Issues:
Aftershock:
Pros - allows tanks to provide some damage in the early game that way they are not useless until they become a quality cc bot/meat shield.
Cons-Tanks are not supposed to be snowball champs, but ones more focused on group fighting. If they get ahead early because of aftershock's damage they become a monster in the mid game as their cc and beefiness comes online, but the enemies have not outscaled aftershock damage yet because they are only like lvl 9-10.
Electrocute:
Pros-allows assasins to scale better into late game so they have a higher rate of success against squishies, and they can even get fed off tanky champs since it will take longer for a tank to build enough defense to survive this level of burst.
Cons-longer cooldown than thunderlords so it's not great for in lane trading but is still better for planned all ins (which is what assassins are built for).
All the damage available in secondary tree:
Pros-provides a very clear difference between someone who wants to just get through lane phase versus someone wanting to push the lead and be aggressive.
Cons-if you go defensive or utility options in second tree you are giving up very noticeable amounts of damage (think of things like gathering storm and burn options in the sorcery tree, or cheap shot/lethality buff in domination). This means that if you go a defensive option in your secondary tree in order to get through laning and scale up, it takes even LONGER to outscale because you're not just fighting the enemy's xp and items, but their passive damage gain through runes as well. In some matchups it might even take huge chunk out of the value of scaling (think like a Caitlyn going Precision, then taking Gathering storm and Celerity in the sorcery tree as her secondary, then Tristana on the other end goes Precision and then defense for second tree. The outscale of trist is Much less noticeable at the post 30 minute mark.) Let's not even talk about how far behind you get set back as an ADC if you go ninja tabi over zerker greaves in the current state of the game.
The nerf to base defensive stats:
Pros- makes early game burst stuff and poke more viable. They have a larger window for success because it takes longer for them to get outscaled. Getting multiple tanks with a scaling ADC is less OP.
Cons- not only does burst damage feel overprioritized in the game as a whole (both all in and poke style), but even if you are playing an early game champ who has their own burst, if you get even a little behind to like a Maokai you feel like you can get clapped cuz you're squishy as hell.
Summary: These are only a few of the issues but I've listed some big ones so that this novel isn't even longer. There are a lot of if's and but's in here. Everything feels like a gamble. Everything is great if you do well early, but feels horrid if you get behind early. Your teammate feeding their lane is even harder to carry through. You get punished too hard for every little thing. It feels like the only reliable strat is going for really strong lanes, or going for stuff that has really good power spikes in the early and/or mid game. Scaling comps, team fight comps, disengage/re-engage comps, sacking a lane with a low econ laner like a utility mid or tank top, all these feel really hard. Huge swaths of champs and strats are being edged out, windows for outplay and out building (like going ninja tabi on a adc to survive lethality building enemies) are getting smaller, and one-sided games are as boring as they have ever been (whether you are winning or losing). I'm the type of player who strives to play as cleanly as possible from beginning to end no matter how the game is going (and I think there are a lot of us out there). By the time I get so far ahead that I feel like I don't need to play cleanly I'm ready to move on to the next game, and I don't want Riot to make it easier to close and the average game time even smaller. I want Riot to think more of tug o war and less of pushing over a line of dominoes.
SOLUTION: remove aftershock in favor of another option that is more in line with the "tank" identity rather than just a raw power buff, then give everyone back roughly the same amount of base armor and magic resist they would have had under last season's rune and mastery system. This way bruisers and assassins only snowball really hard if they are being mindful of their windows and playing consistently well, not just playing well early on then playing like a monkey cuz they are snowballing so hard off stats/damage synergies.......AND tanks won't clap everyone without a defensive mechanic in the early/mid game. BOOM, people can stop complaining about burst damage being so strong AND those snowball damage champs can stop complaining about losing duels to tanks who are supposed to be team cc bots not duelists. AND AND AND, less troll comps because the removal of so much snowball potential means that people picking all AD or all AP comps is much more punishable since their window to snowball is smaller! And play patterns become more unique based on champion selection since it will take longer to get to the point where you can just run at the enemy constantly (so if you're playing a spit push champ like teemo you will actually have to split push for a large portion of the game like you are built to do, not just walk up to a fight and q someone for 80% of their hp because your team got a 4 kill lead with first tower blood).
etc which are all tanky items that compliment their horrific base damage.
is considered a "juggernaut" but he has a built in shield, a slow and two hard CC's. I think people build him full tank as well.....but apparently.....he's not a tank. [sg-lux-2]
is a tank and all he has is a speed up. When you capture a spire 
is considered a tank even though he has no mobility other than buying 
. Cho only has one hard CC on top of everything with no "real" reliable engage, yet he can fuck up a carry on a level better than even 
can. might I add, without even building any damage items either
" dw , im picking a tank" : 
."
