It's no secret that the vast majority of the community does not care for the ability fungal growth. I am not going to make any claims on the balance of this ability - whether it's ridiculously OP, "balanced" perfectly as is, or is utterly necessarily for Zerg to function and not get stomped. These concerns are wholly irrelevant, as they could be fixed with other balance adjustment.
The entire disdain for the ability comes not from the splash damage, but it's ability to root units and cause a chain fungal that can kill your army without you ever getting to utilize/micro it, which can be exceedingly frustrating to play as and is undoubtedly boring to watch. As many have suggested on teamliquid, changing it to a slow (at whatever percentage is balanced) would have ridiculous benefits, as it would allow things such as sentry/stalker/immortal to FF off remaining infestors and slowly retreat out of chain fungal range, vikings to split up if caught, marines to SLOWLY push forward to the BL's or at least split if caught by an initial fungal and save ~25% of the troops, etc.
If you're on the train that fungal is OP, this might be a good nerf. If you're on the train that it's not OP and just a !@#$ty ability that's necessary, than remember it can be compensated with OTHER abilities, such as the one that's following...
Fungal growth negates usage of all passive/usable abilities/techs. This would not negate already activated abilities. For example, if you stim prior to being fungaled, your stim would not be cancelled. However, you would be denied the ability to use a future stim during the four seconds you're actually fungaled. So if you can fungal a batch of unstimmed marines, they will not be able to stim. Same thing if you manage to get every single sentry with your initial fungal - they cannot subsequently FF at all, meaning the Zerg units could rush in or a chain fungal would still be achievable. Although if you don't get all the sentries, the ones not fungaled could FF off the infestor army and bail from the chain fungals.
These are the abilities it would affect. Depending on how strong the ability actually ends up being in gameplay, you could consequently adjust how much slow the fungal actually causes (if it ends up not really being that useful, then make it still slow at 80%. If it is super strong, make it only slow 30% or even remove the slow!).
Well %^-* Blizzard's max character limit per single post, I'll post the specific abilities it would affect in the subsequent second post. Another alternative to consider people are talking about on TL is to have it reduce the rate of fire by x% (of course, subject to balance) while also changing it from a complete stop to a certain slow percentage.