Very diverse, a lot to explore, but that's pretty much what makes Ragnarok Online what it is.
Grinding, endless grinding. If you can't stand extreme grinding games, stay away from this one. You'll have to spend hours and hours everyday for a very long time to reach the maximum level. Even after reaching the maximum level, you have the option to get "reborn" to start grinding again from level one with a few extra stats and the option to choose advanced versions of the old classes later on. Getting items is a pain, a lot of simple items drop with impossible rates, it's not surprising to see how a lot of people use bots to get them. As if it wasn't enough, Gravity keeps on emphasizing the grinding aspect of this game with their patches which is, in my point of view, the main cause of its misfortune with bots.
Hunting may be fun for a while - if you are playing a magic user - but after a while it gets dull. Magicians are fun at the beginning, there are a bunch of spells to unlock with different effects, such as freezing, petrifying and some others, which make them somewhat fun to play at the beginning. Later on when all your skill points are used, leveling with them gets a bit boring, but they're still less linear on this aspect if compared to the other classes. Acolyte-branch users will also experience a better hunting experience, since they can heal undead monsters to cause damage and support other people, they also have the option of choosing "exorcism" spells, made specifically to hunt undead monsters. For the other classes, saving some exceptions (boss monsters), you'll basically have to click on your enemies and wait until they die for a good time. Even for Magician/Acolyte branches it gets boring and repetitive after a while.
Battle is all about clicking on your enemies, but there are some interesting points about it. I can't say that this game requires a lot of skill to be played, but it certainly uses a lot of strategy. If you know monster elements and how to build up good card combinations you'll most likely prevail among other players (that is, if you get access to the cards, of course), the strategy engine behind the battle system is definitely well elaborated. PvP doesn't require much skill either but strategy also plays the main role behind it. In the end you'll realize that PvP isn't really special if compared to other games, but it may be fun to play for a while. WoE (a.k.a. War of Emperium) is the guild battle system in Ragnarok Online. It isn't much different from PvP, but it adds a task and emphasizes teamplay. Differently from PvP, I think that WoE can be really fun if you play it with good friends.
I can't say much about the community, since it depends on which server you're playing on (iRO, kRO, bRO, etc) but still, it's pretty clear that all of them have a considerable amount of bots, just like most of the other popular grinding games, which can be an annoyance sometimes.
Yes, this game is addictive, but if you notice, people who are playing barely stop to enjoy the present moment of their gameplay, because it's pure grinding. They usually expect to get more experience and items to have fun in PvP/WoE, or they just want to explore the classes and skills. Generally, picturing themselves happy with what they can earn in the future gives them motivation to play. The fact that you can find something very rare all of the sudden from simple weak monsters also gives motivation to some players to keep on hunting. It's like a gambling machine, it's addictive if you hope you'll get a jackpot. But differently from a gambling machine, it doesn't get much better if you successfully get to where you first expected, which is kind of disappointing. At least that's my point of view about it.
If you can cope with the level grind and decide to play this game, make sure to bring a few friends with you and get ready to spend days hunting. It will be boring if you have no one to share your adventures with, but even alone you certainly will enjoy exploring the diversity of Ragnarok Online.