Pop cap wants to know the 3 variants to buff/nerf via twitter (link inside)

Sunflowers need some love (buff).  Hard to support and heal your team when your weak and taken out so easily. Give them more health and cc defense. Nerf Imp and super brainz. I think the problem may be too many imps on a team at once. Team with a bunch of imps, scientists, and super brains is not easy to go up against.

So instead of parroting some of the points others have made I'll throw in suggestions on how things can be buffed/nerfed

Chomper -- Lower CD on Goop and his cannon; higher HP OR Speed OR bite damage; no speed reduction on burrow bite OR shorter chewing. Seriously the chomper is the only class with no decent range. Throw him a bone and let him be OP if all this is too much.  No one should want to get even remotely close to a Chomper. Being near him should give Zombies nightmares.

Sun flower -- Needs new and better offensive abilities. Sunbeam should have damage mitigation OR a buff. Hell any plant rooted should get a damage mitigation if Engineer and Deadbeard gets it. Healing should be on par with scientist who has 2 abilities for heals AND high damage. Come on!

Cactus -- after seeing DeadBeard.. yea give her something. Maybe an ability to throw grenades? How about some better damn walls that block splash damage? A better drone?

Rose -- Give her warps and HP buff. That'll fix her right up!

Corn -- Needs more speed or HP because he's very close to All-Star ability wise. Everything else is fine

Citron -- Where do I begin? Stronger shield, how about one that doesn't let splash damage through. How about one that can let him survive a direct ZPG like the trailer shows? How about a stronger beam that is better than SuperBrainz cause all he has is beam. Superbrainz has melee for up close. Let his Roll attack do 100 damage and yes it'll kill an imp. Any Imp hit with that ability that has a 1.5 sec tell (charging roll) should die anyway. Give E.M Peach a longer stun. 5 secs? Just longer than the Engineer's stun would be good.

So.. nerf.. zombie side... gets it all.

Scientists shouldn't be a death machine. Though to fix this you'll have to fix the Plant's crit hit box which is their massively sized heads.

Nearly all plants have one and because of it they should generally have higher hp or smaller crit boxes. What Scientist and DeadBeard prove is that there is a massive crit hitbox issue with the game. As with any shooter, head shots do extra damage but many of the plants have their heads as their center of mass (Sunflower, peashooter, citron, and chomper). Why is that an issue? Because when you aim at a far enough target you aim for center of mass incase you hit. All of Zombie's center of mass is at the tops of their bodies. DeadBeard actually gets an advantage in this because he's thin and tall so you have to aim higher. The imp gets a disadvantage but is more like the plants in that regard.

SuperBrainz needs a nerf but where is the tricky part. Beyond Electro Brainz's damage nerf. His twister needs a tweak. Maybe not to pull plants in? He can can already run down half of the plants with that. Charge in and twist is too easy. ]

Honestly I think the only zombie who is under performing right now is the foot soldier because he's a harder class to master than the others.

Now beyond all of this? Sure plants are more CC oriented but that CC needs to be stronger. Their need alternative abilities that let them take offense than play defense. Too many of the plants abilities are defensive.