EPT1. A LOT of new interesting ways to do different strategies, ie. not only being able to go Infestor Broodlord in ZvP.
2. New ways or Zerg to contain/pressure Terran and Protoss, primarily in the form of the Swarm Host, but also by using the higher spped of hydras to have an mobile Roach Hydra army versus Mech and Protoss.
3. More opportunity to od what you want to do strategy wise such as going for Swarm host contain into mass expand ( my favorite ZvT style in HOTS).
4. The fact that players are being more creative as a result of a meta game not (really) having formed which reveals interesting uses for both the new untis but also some of the older units.
Hope you enjoyed my first blog i will hopefully make more in the future as the will to od so sohws itself !
Disclaimer: i am VERY biased because i primarily play zerg so anybody who dsiagrees i would love to hear your opinion!
. They just can't function in pairs...not enough DPS. It's an issue with proxy units. As long as the SH remains as it is design-wise I won't really be satisfied with it - if you watch a lot of HOTS streams you see that zergs basically use it as a more effective broodlord to produce contains (with spines and vipers/corruptors). 
