Sicarius
Reggie J Worthington
well thats the thing i'm approaching at. where the folks in angs time left off in season 3, the folks in korras time picked up.
kitara outlawed blood bending, meaning it wasn't JUST her that was able to bloodbend, there must have been several others. yakon was unusually powerful at it though
lightning bending was demonstrated by uncle iroh in season 2(i think?) as a very hard technique to achieve. as such, then you saw zukos sister doing it in season 2, and then zuko in season 3, along with his dad. no one else. yet in korras time, as mentioned above, electricity bending has become fairly common where people are electricity-bending generators and they are being used in tournaments as 'professional' benders. professionals are not masters.
flying technique that fire benders did, i think uncle iroh showed that, then fire lord ozai. even ang wasn't able to do that. ang flew using water bending and/or air bending. yet general iroh can easily do it
metal bending became easy to do after toph did it too.
point being, what exactly were the previous avatars doing for centuries? just doing the basics? at the same time, the bending in angs time took a lot of concentration to do, and usually happened when they had a specific need to do em (like korras air bending when she needed to do it). yet it seems that either everyone in season 1 of korra is a master (which i highly doubt as if everyone is a master, it dilutes their powers) or they are going to expand on these techniques and learn new stuff.
point being, that techniques took centuries, maybe even millions of years to learn, yet the time difference between ang and korra demonstrates that bending everyone can do master techniques with ease.
also, when aang was told that he was the new avatar, he ran away. even at first season, he was having a hard time bending water and earth and even fire. it came naturally to him, but still it was hard. however, it seems that korra learned all the basic bending stuff by the time she was 1 and a half years old: basically being born with it
Or it's just a matter of being taught how to do it? All it takes it practice.
Any Water Bender can be taught how to Blood Bend, and healing, if given the time to learn.
Same with Fire benders and Lightning. It's part of fire bending, but to be able to do it, you need to be taught.
It's not that these people are masters, or using master techniques. It's just that as time passed, rare techniques become more prevalent due to need (lightning, and metal benders), or still just as rare (Blood bending because it does take a master to teach it, and the only one left is Kitara so far)