1) The combat and weapon variety: You get to choose your playstyle. Want to use guns? Have at it. Traditional samurai who shall not dishonor themselves? Use bows and shuriken's. The combat has a stamina bar and plenty of skills to learn and combat styles to master as you play. The more you use a certain weapon, the better it gets. The closer you get to your friends or hunt down criminals they will give you new combat styles and/or teach you the next level of it.
2) RPG system is great and allows you to focus on what you want. With skill points you can learn anything across the board (Strength, Dex, Charisma, Intellect) however, there are individual points for each Special stat so you can learn specific things. It works great and makes you want to seek out more points through shrines and do quests to create your perfect warrior/mercenary/ninja whatever you want to build.
3) Loot: This has been critiqued quite a lot here and all I can really say is the system that is in place will constantly give you new weapons, guns, bows, etc as you play. This is to encourage players to try out new weapons. If you come across something that doesn't work for you, break it down or do another mission. Money is nice but not 100% needed most of the time, break your excess weapons and armor down for more materials so you can create items to aid you in battle like bullets for your pistol/rifle or grenades and much much more. Loot also has bonuses that wearing more increases diff stats
4) Customization: Very much like the weapon variety in the game. The outfits in the game are top tier. There is plenty of variation AS WELL AS TRANSMOG to let you create the badass character you want. I was easily able to create a western merc who uses a bayonet (melee musket) and katana and then I created an awesome looking samurai. You can save your favorite transmogs as well to go back to any look you have previously created. Most games struggle to even give you this at all if not have a year later, so Team Ninja should be given massive props on that front. There are even sick looking armor sets that can be unlocked not only from exploring the world and completing quests but also getting closer to your allies.
5) Allies: Is it the best AI I have seen in a game? No. Do allies aid you in battle well enough so you can take out the enemies that need to be dealt with while the other enemies go towards your allies? Yes yes yes. You can buff your allies with gifts that will make them better in combat and when you get closer to them (bond system) they will give you awesome gear and it will buff their stats as well. There may be some allies (very few) that aren't as interesting as others but they each have their own story and reasonings to be on either side of the conflict. They are incredibly interesting and there quests flesh out more of the story going on in Japan as well as the allies themselves.
6) Story: If you are like me and loved the movie The Last Samurai then you will find a lot of this story vastly interesting and will cling to the characters you meet. I gripe I have seen from the community is how many times you will fight someone who is your ally out of nowhere and found that they did not like that, however; this is a very VERY common troupe used in Japanese games stemming from Final Fantasy, Yakuza, and even NIOH where no one complained about it. The story lets you work with both sides of the conflict (Pro or Anti Shogunate), you have plenty of time to choose which one.
Get this game! To be quite frank, this is a perfect example of a game that is getting bad reviews over things that in my opinion are either ridiculous or way too insignificant to warrant a low score.