If I was limited to answering this question with just a few words I would go with Content, Cooperation, and Realism. However, since I'm allowed to write as much as I would like let's go into more depth on each of these. First though, a quick overview of the series.
The original Monster Hunter was released on Playstation 2 in 2004. A simple idea for sure. Large dangerous creatures roam the wild and you go out and hunt them. That is pretty much it. But that concept has grown in to a total of 10 title releases and 4 spinoffs (with a 5th yet to be released). The series is wildly popular in Japan and is steadily gaining a following in the West, which is good news for hunters in EU and NA. So what is it that makes the series interesting? It's time to develop those three words, the first being....
Content. It is massive, actually it is more than massive. The game utilizes weapons, armor, armor skills, items, gems, charms, and on and on. Each of those groups have almost endless sections within them. There are 12 types of weapons (in Monster Hunter 3 Ultimate). Each armor set is made of five pieces that can be mixed and matched to produce a desired defense or armor skill. In order to forge these weapons or armor you need to collect the required items. In the aforementioned MH3U there are 1080 collectable items. Some are very common, some take a lot of time to get.
The different weapon types offer the player the choice of what is important to him. Some, like the Sword and Shield or the Dual Blades favor speed, while Great Swords and Hammers deal more damage but aren't nearly as quick. Bowguns and Bows offer the advantage of dealing damage from a distance at the expense of higher defensive armor. Hunting Horns allow you to provide boosts to your allies, giving them extra health or defense during the middle of the quest. I personally have always liked the Switch Axe, it gives descent damage, not as slow as the Great Sword and I can change between smashing the monster's head or slashing on its tail with a press of a button.
Armor Skills are just as varied. Some make you impervious to heat or cold. Some help you swim faster or affect how much health you will start with. Eventually you learn that you will have a different armor set saved for different situations. If you are gathering items in the Volcano you will have a set built to give you Gathering +2, Speed Gatherer, and Divine Whim which allows you to gather more items quicker without breaking your axes and bug nets. But those skills aren't very helpful when you are actually fighting a monster. For that you might want High Grade Earplugs and Sharpness +1, or if it is a capture quest (which you fail if you accidentally kill the target) you will want Trap Expert and Capture Guru. The combinations are endless and it all comes down you knowing what you are wanting to do and what will help you do it.
After spending some time in the game it becomes clear that you won't get very far going through it blind. Know what you need is one thing, knowing how to get it is something else. One of the harder to obtained items from Monster Hunter Tri was the Skypiercer which you could get from defeating the Alatreon. Assuming you just had that piece of information you could go out and beat up on the Alatreon over and over again without getting that all important piece. Doing some research (or more than likely asking one of the other hunters in the game) you then discover that in order to get a Skypiercer you have to break both of Ala's horns. Even then you only have a 15% chance of getting one, but that is better than what you had before. Asking for assistance brings us to the next point...
Cooperation. In today's multi-player online games, nine times out of ten you are playing competitively against others. You are trying to fight, shoot, or destroy them. For some, there is a level of anxiety about playing with people they don't know. We may question our skills, feel that we aren't really that good. So when we play against someone, our biggest fear is that they would confirm that thought. "Dude, you really suck at this. Go play with other noobies!" is the last thing you want to hear, esp if you are just starting out. The social aspect of Monster Hunter takes the online multi-play into the cooperative mode. Every journey starts with the first step and for some reason the players of Monster Hunter remember taking that first step themselves. We remember when we were helped to take down a tough target. As we get better, we want to help others. It is not uncommon to be playing with someone that has a fully-fledged high rank weapons and armor and then hear them say, "Ok, I'm going to go find someone that needs help." That is one of the reasons that Monster Hunter Tri did not leave my Wii for almost 2 years.
The cooperation aspect also transcends the game. Players enjoy helping each other and sharing information. That has resulted in active forums such as Capcom's Unity and the fan run Club1kJho.com in which players can ask for help, share stories, or post tutorials and resources to aid their fellow hunters. Yours truly has spend a fair amount of time building the weapon trees for the MH3U armory that shows the paths and required items needed to get to those powerful weapons you would like to have in your hunter's storage chest when you are prepping for a quest.
Ok, so a game that places you in a fantasy setting, with impossible weapons and armor, fighting ridiculously unbelievable animals would hardly qualify as being Realistic, yet anyone that has hunted in real life know two things. First, to have success you need to understand what you are hunting and prepare for it. Second, no matter how much you prepare even the most experienced hunter sometimes comes back empty handed. Such is the case with Monster Hunter.
Each monster has certain attacks and weaknesses. Knowing the monster's attacks helps put you in a position to take less damage, and thus spend more of your time dealing damage of your own instead of finding a safe moment to down a Mega Potion to recover from the tail swing that sent you flying. Knowing that a particular monster is weak to Fire helps you choose the right weapon. Two weapons with the same Raw damage but different elemental effects can have a very different result in the field. Just like in real life, the educated hunter is the successful one.
However, sometimes you can do everything correct, make all of the right moves, down the monster in record time, and still not get what you are needing. This has lead to the urban legend of the "desire sensor." Players that have been farming a monster trying to get a particular item will often comment that the desire sensor has kicked in. The game somehow knows what you need and refuses to give it to you. There is of course no truth to this (or so we are told) but sometimes it is hard to believe. Case in point, I recently set my sights on upgrading my Gigant Burst Switch Axe to the Giganceladus. In order to do that, one of the items I needed was one Diablos Hardhorn. Checking the handy MH3U app on my Android Phone I see that I have a 5% chance of getting one if I break the horns of a regular Diablos and a 7% chance of doing so with a Black Diablos. I find a room online titled "Hardhorn" and the listed target is "Diablos" so it looks like I'm not the only one interesting in getting this item.
Statistically, with a 5% chance I should have one within 20 monsters. One of the available quests is to hunt 3 Diablos' in succession, that means that worse case scenario has me getting one on the 7th quest. Then the desire sensor kicks in, and after my 29th quest I get one. Sometimes in life that happens and it may seem frustrating. Let me amend that....it IS frustrating. But on that 29th attempt when I see it listed in the quest rewards, the 28 previous quests melt into satisfaction. I sent out on a goal and I achieved it. It almost is as if I enjoyed it more because it wasn't easy. Does that sound like something your parents told you when you were growing up? Along the path of those 29 quests totaling 60 or so monsters I met several good hunters that taught me tricks that I then shared with others. I have more Guild Cards traded with players and thus more players that I can call on for help later on.
The only thing that I wish Capcom would have included in the MH3U version of the game was the ability to post screenshots in Miiverse. After I got that Hardhorn I would have loved to post in Miiverse with a screen capture of it. Standing in front of the defeated animal for a picture to share with others smacks of real life hunting. Maybe it will come in an update, after all Capcom to released a patch that combined the EU and NA servers so players on both sides of the pond could quest together. That has been extremely helpful as many of my hunting parties are made up of EU hunters. In the meantime there is plenty to keep me busy as I try to upgrade my Water, Ice, and Exhaust axes.
As for those Skypiercers and that even harder to obtain Skyswayer, I'm coming for it Alatreon. It may take some time but it will be mine....oh yes, it will be mine.