Add a new statistic (Average Card Cost) to the mana curve graph UI simply displaying the mean average cost of card in your deck.
I would like to tell people something like "My ACC was 4.9, but after some tinkering I got it down to 4.2 to better respond to the early game aggro meta. I did that by turning to ethereal conjurors into water elementals. I had to do it because the Warrior Pirate netdeck is 2.7 ACC and I just had to speed up my midrange mage".
I feel like that statistic AND slang is needed because those clunky 7 bar graphs (like the one on the bottom right hand of the HDT UI) showing your mana curve is difficult to discuss in depth online in the forums or on Blizzard Chat. All you can say is things like "mid range" or "my deck has lots of cheap minions" or whatever to get your point across. Having an ACC stat would make communicating your deck range more very clear and simple. This is especially the case when you've made some changes to your deck to make it faster or slower, yet still stayed within your general midrange or agro standard constraints. Like the example at the very top regarding a mage lowering his ACC.
It's also a Quality of Life improvement to the HDT User Interface as well. Yes, having a simple number represent your deck is not a super pro analysis tool. It IS good to see those entire 7 bar graphs so you can spot any gaping holes in your curve, and then also adding them all up in your head and dividing to find the average. But sometimes I don't want to look at that and then do algebra in my head, I'd just like to know one darn number with the average cost of all cards in my deck.
So my suggestion is that we essentially create a new statistic, and add it to the HDT user interface overlay right next to the mana graph in the form of a simple number (i.e. 3.3). Frankly, I think this would be a good trend to set for the entire community and that all mana curve graphs from here on out should integrate that ACC number and display it in a corner of the graph. This new statistic will be very useful for beginners to easily understand the concept of deck weight. And it will also be useful for veterans who are discussing or analyzing minor changes.