What is the most annoying thing about the messaging service? For me, the inability to quickly post a reaction to someone's message, forcing me to either send an emoji as a separate reply, or write a largely unnecessary response, is the reason. The feature is present in many other chat apps, but not on WhatsApp.
This may change in the future. The reaction feature has gone live for some users who have installed the latest version of the app on their phone.
The feature is similar to what you get in other chat apps, with the exception of the surprise face.
Only a small group of users are getting this option. It is much more frustrating for those who can't use them if everyone is able to see the reactions.
There is no guarantee that the feature will be available to everyone, but it is likely that it will be soon.