EPTTo understand this, you must look not just to the new units, but to the availability of casting units within the race. Now, my definition of a casting unit is one which has an ability that does damage or effects a battle, not something that does damage of its own accord (note this definition is really only for zerg casters). In that regard, the infestor, the viper, and the swarm host are all casters.
The first point, however, is that these units will make the learning curve for zerg very steep, requiring people to master these units before the zerg race is fully unlocked. However, once that has happened, zerg will be able to master each of turtling, harassing, and full siege attack, two of which zerg was unable to do successfully before.
A few quick hypotheticals to make this point. Use of the viper (even though it requires a fast hive tech) will make siege lines less powerful, pulling tanks either closer or out of range. Furthermore they can reduce range on bio units, allowing you to move mutas more easily. Picture this: a solid marine tank combo that is sitting outside your natural or third. You have: mutas, a few vipers, and a few infestors. You fly the vipers in and throw down some binding clouds around the tanks or immediately pull the tanks away. Either way, your mutas can have at it. You could also safely burrow in some infestors for some infested terran friendly splash damage.
Another situation is against a protoss opponent. One of the most lasting problems when facing a toss enemy is facing a death ball and how to beat it. When the protoss player has maxed out with an army composed of stalkers, sentries, colossus, and sometimes a mothership, it would require a lot of either broodlords or ultras supported by a good deal of infestors. It also requires forcing the attack in a place that is most suitable to the zerg player. However, with the use of the swarm host and viper, we could take this army down piece by piece. First, we can set up a line of swarm hosts, similarly to a tank line. Then, our army of infestors, vipers, and hydralisks can sit near that line, as we await the engagement. When it comes, pulling colossus into the hydras will kill them quickly enough to not sustain damage (using infestor fungals or overseers for necessary detection) and then taking over the army with the dps of hydras and some tanking by locusts.
Some added concerns would obviously be terrans increased use of mech because of the new available units. However, the warhound would not be too useful against zerg's bio forces, and the battle hellion could not hit mutas in the air. The use of spiders mines is certainly a threat to the low health mutas and will have to be dealt with carefully so as to not lose many, or any mutas (quick splits or hotboxing might be useful).
Protoss adds the use of the tempest, which can utilize long range to knock out siege lines, probably aimed at tanks and swarm hosts. But still, zerg gets the upperhand here, having vipers that could pull these units out of range or into the fray in the event of a retreat. Also, adding ultras with a burrow charge will help immensely, tanking damage from tempests and hitting the protoss army ferociously.
As previously stated, the expertise and precision required to execute these tactics has a steep learning curve. But once players can effectively utilize the new casting abilities, as well as some help from the new hydra and ultra upgrades, zerg has a powerful new army.
