I'm at HR7 at the moment. This Monster Hunter game has proven to be as addicting as ever. At the moment I main the Insect Glaive, Sword and Shield and Greatsword, in that order.
6* Quests are actually somewhere in between low rank and high rank. Some of the monsters you fight at 6* will drop high rank materials, but the quests themselves are still considered low rank. Additionally, you will always start at the base in 6* quests, which is a low rank feature.
The difficulty amps up considerably in high rank, both online and in singleplayer. I had fainted maybe two or three times total during low rank quests. When I got to high rank, though, I started having much greater difficulty with the game. Monsters such as pink rathian just tended to destroy me.
After getting better armor, though, it became a lot easier. I currently have several different armor sets that I alternate from. I actually prefer to have a few different sets as they give different elemental resistances and skills. Tetsucabra is a great set for low rank but it becomes quickly obsolete later on in the game. This is true even with high rank Tetsucabra sets. A problem with the Tetsu sets is that Health becomes a useless stat once you are able to acquire max potions and get better ingredients for food.
Some sets I like:
Lagombi: There's nothing great about this set. I just like the design.
Najarala: It gives you more materials when you capture monsters as well as Earplugs/HG Earplugs. You can gem in perception and focus on capturing monsters. It also has high fire resistance.
Rathalos: I don't really like the design of the Rathalos set, but Attack Up is always good.
Arc (Shageru Magala): I like the design of this set. Like the Lagombi, it's unspectacular, but the defense is pretty good.
Red Khezu: This set is really ideal for SNS. Wide-Range is a fantastic skill, especially online. I also think this armor set looks great, except for the hat which I replaced with the Caravaneer's hat instead.