Wow, Bioshock Infinite's horde mode takes the "annoying center text" to a whole new level.
KILL +10
VIGOR KILL +25
DAMAGE OVER TIME +30
DOUBLE KILL +25
etc...
If there was an example of where medals work better, this would be it. Firefight is simply a better mode; the elements of the challenges and the global leaderboards are something that 343 should look into, though (the leaderboard in Reach might as well not have existed.)
Offering Spartan Points or XP or credits or whatnot and allowing people to unlock concept art (that you could screenshot out for wallpaper or etc.), additional bits of Halo lore, and character models seems like it would be a good addition to a Reach-style armory system, alongside the gun skins and armor variants (bring back armor effects too!) Reach's system felt like towards the middle and end that you were forced into saving for long periods to actually get what you want, having more content to unlock means you can sprinkle out more of interest while the overall level progression stays the same.