According to Ben Brode, the game's alpha featured 4 or 5 keywords which were removed from the game prior to the beginning of the beta.
Keywords were specifically designed by the developers to help players to quickly recognise when a large number of minions have exactly the same ability, such as minions with Taunt, saving time spent reading each individual card's text. However, while keywords make it easier in cases where they are frequently encountered, when such examples are rare, keywords can actually serve to make understanding cards harder, requiring players to mouse over cards in understand to examine their abilities. The case of the Poison ability was cited by Ben Brode as an example where the developers could have added a keyword, but chose instead to simply describe the ability in full, since it is was originally only found on 2 cards. Senior Producer Yong Woo explained:
"You know, keywords are double-edged swords. On the one hand, it really condenses information. I tell you Deathrattle and you know what it is right away. I tell somebody off the street Deathrattle and he is going to be like "What are you... selling me a snake?". They will not know what I am talking about, so it is a balance we have to reach between condensing the information and making too hard a set of vocabulary for new players to understand. It is something we continue to work on."
Ben Brode has also stated that when a keyword conveys more than one behaviour rule, it can be hard for players to remember these from just the keyword. For example, Taunt and Charge immediately convey that the minion possesses a single, specific ability. In contrast, the "elusive" ability is tricky because it has more than one effect, protecting the minion from both spells and hero powers, but not Battlecries or attacks. Ben Brode has stated that the elusive ability was "right on the razor's edge" regarding whether or not to create a keyword for it.
Yong Woo has stated that it is possible in the future that keywords may be added for currently existing abilities such as the "elusive" ability of the Faerie Dragon, Laughing Sister and Spectral Knight - should there come to be a sufficient number of minions with that behaviour in the game. Other card text for which keywords were considered, and for which keywords may be added in the future, include on-draw effects, Poison, and Joust.
Following its launch, the game did not see the addition of any new keywords until The Grand Tournament, when Inspire was introduced. Inspire was initially represented by the text "Whenever you use your Hero Power do X", but the developers felt that replacing it with a new keyword would both save space on cards and also highlight the new mechanic as a key feature of the new expansion. They also felt that the game had been around long enough that it was acceptable to introduce a new keyword. However, the developers did not choose to make the same expansion's new Jousting mechanic into a keyword, partly because of a lack of synergy with established keywords, and partly because it is "more in depth" than other keywords. Inspire also featured on a larger number of cards than Joust.
As the game grows, the developers feel increasingly open to adding new keywords. When the game was new, Ben Brode says, the developers "really wanted to keep ourselves in check as far as the number of keywords we added and how quickly we added them", but since The Grand Tournament have begun adding keywords quite regularly.
The one exception to the general rules for keywords is Mega-Windfury, a keyword added with Goblins vs Gnomes for just one minion: the uncollectible V-07-TR-0N, summoned by Mimiron's Head's triggered effect. However, this keyword is followed on the card by the italicised text, "(Can attack four times a turn.)", essentially negating the use of the Mega-Windfury keyword. The redundancy can be explained by the obvious similarity between Windfury and Mega-Windfury, with the use of the keyword aiding players in immediately understanding the card's ability.