Text in the image: Realm Record! Guild [Guild Name] led by [Guild Master] killed [Boss Name] [Raid Difficulty] in [e!
The above is an image of an announcement that was featured on Lordaeron during its progression 8 years ago.
I suggest that this be featured on Onyxia, as well.

What it was: Every time a guild newly occupied the 1st place on the PvE ladder for any raid boss of any raid difficulty, It would be announced to all players online at the time, in the chat, just like Realm Firsts are announced.

Only one guild got realm first LOD, but how many had the fastest kill in the hours, days, weeks, or months that followed? I would argue quite a few, and each time it happens, it is quite exciting, not only for the guild but for the whole realm.
There are a number of arguments for it, with the primary one being how positively it affects the PvE scene, primarily in boosting competitiveness.
Although the armory features a ladder and a leaderboard, in order to see them, players have to actually want to visit the armory ladder or leaderboard to look at them. Having the records announced in-game, to everyone online at the time, definitely boosts the competitive spirit.
The only other announcements are of Realm Firsts. Realm First achievements simply don't happen often enough to have those announcements contribute to the livelihood of the realm. On the contrary, once the achievement is announced, some people no longer feel that the content is fresh and lose motivation. Although I make this argument, of course I am NOT suggesting to remove Realm First announcements.
Lordaeron is not as alive now as when it was with the release of ICC partly because it WAS cleared and is no longer fresh.
Realm records, on the other hand, are announcements that can run perhaps for as long as the realm has a live PvE population.

Thank you for reading this!