I'm not going to be too hard on Maple Story, its roots date from 2006 or something that old and the game truly evolved through the years. Everything about it got better over time, from how classes started out - at first you had to roll randomly your stats and then fight for 10 levels with a really weak and boring character, while now you just need your core stats and you can skip the tutorial - to the distribution of quests by levels, to the 'balance' of skills to the amount of bugs and random stuff broken here and there. It's not really a good game even tho the concept of MMO platformer is pretty unique but I've spent way much time that I would've with this game during many years.
The first years were kind of insane
The first years of Maple Story where when I spent money on the game (To buy Double Exp cards, for instance, or pets that would loot items for me, a tiresome task) because it was the only way to move forward in a timely manner. There were no quests to do, pretty much all you could do was grind, but I grinded and grinded and did little events here and there to level up, playing with my wife. They added some quests where you had to kill thousands of monsters or anything equally ridiculous so after a long while, I stopped playing. I might have spent more than 100$ in Maple Story, but there's still stuff I could pay for if I really wanted to, which brings me to my first problem.
Maple Story is pay-to-win...
The cash shop is full of items you would need to be #1 in MS since everything is skewed against you. Travelling takes forever, but you can buy teleportation rocks. You can get your skills up to some level, then you need to find an in-game book with a random chance of working or just buy it for real money. You need scrolls to upgrade your items but they have a chance to not work and a chance to destroy your items forever so you can buy scrolls to give them a better chance to work or to protect your items in case they break. You can even buy things that will let you equip certain items you can't yet. You can buy exp, money, pets that will loot for you and heal you when your health dips too low, you can also buy tons of cosmetic stuff too, but it's not really the focus. AP (stats) rests, SP (skill) resets, name changes, additional character slots...
The problem is that there's always more of it, you can always want to buy more stuff, it's the bread and butter of free to play games, but there's just too much stuff they want you to pay for. Stuff that might make the game actually fun. Being able to teleport wherever you want, having a way to loot automatically items, having more inventory space, leveling up faster... Seeing this locked behind a money gate is frustrating. Oh, and did I mention you lose experience when you die? I think there's a cash shop item that prevents that!
How I would fix this
I know this isn't a very popular idea, but just sell a 40$ copy of the game. Call it the deluxe edition, throw in pretty much everything a Maple Story player would want and offer everything else at a discount, being able to play with all of the bells and whistles unlocked from the start, that way you may have players buying this and enjoying the game more because now they have more options.
...but there's nothing to win.
MS isn't "balanced" in any way. When they release a new character, the new character is always better than the old ones - otherwise who would want to play as them? - and they have nice event quests and rewards and bonuses that you wouldn't get with the old classes if you played them. The enemies follow a quite linear curve of difficulty, you'll get hit for about the same percentage of your overall health whatever you're fighting if you stick to monsters your level. The stats systems are largely useless, there's strength, dexterity, luck and intelligence, the only purpose of these stats is to increase your damage. Why have them at all? Why not grey everything out except your main stat? Because you get gear and items that increase the other stats and sometimes you might be able to transfer them to your main stat, of course.
The in-game economy is full of inflation but it's not that hard to grind for money. Of course, everything is a money sink so having tons of money isn't anything to be proud of, you can also buy some more with real cash. There's also close to no end-game. A while ago, you needed level 120 to promote to your fourth class advancement and get your best skills with some incredible effects and that was a challenge because after level 50 onwards there were close to no quests. Nowadays you get your fourth class at level 100 and then can unlock all of your best skills, furthermore there are no quests after level 110 or so. You need books to get more levels of your skills and they drop randomly, work half the time or less, and only raise the max levels by 10. Ok, the 4th class skills are cool but level 1 exploding arrows will be the same as level 30, only the numerical values will increase.
Besides skills, you can get better gear by doing party quests which involve playing with other people. I'm not sure what's wrong with the audience in free to play games but MS is no exception. So there's nothing to do past a certain level and you can only grind to increase your stats which allow you to deal a little bit more of damage so you can grind stronger monsters...
How I would fix this
As I'm sure Nexon cannot create content faster than people consume it, I'm not going to suggest building higher level content. Adding new classes is a temporary stopgap at best; people will try them out and either outlevel their other characters - because of the added content for that class - or just stop caring altogether. My idea involves restarting your character over when you're at a level where there is nothing left to do. You get to level 120 (or 130, or 140, whatever) and you can restart your character to level 1, gaining something that way. Maybe your skills max levels get increased by 1 automatically? Maybe you start with a few more stats? I know I would've done it.
At least they're trying to get your money
So many events are happening at once in MS, there's always some little thing or another. Right now you can play a board game with dice you gain each ten minutes and depending on the squares you land on you either get new quests or bonuses. There's also an event where if you play 30 minutes you get one special coin. Also another even that nets you a coin for each 200 monsters you kill. Another event going on is that each 3rd monster you kill nets you bonus experience. There's an in-game event calendar for a reason, obviously, and to say that Nexon isn't taking care of MS is not true.
I find it ironic that they tell players "Hey! Don't play too much it can be hazardous to your health!" and then say "Play five hours straight to get 30% bonus experience!" Of course they want you to play too much, because then, maybe you'll spend some money. Some quests even need you to spend money to be able to complete them... And let's not forget the double exp and double drop hours where they just give everyone a taste of what it's like to level up almost decently fast.
All and all, MS is a cash grab bundled around a weird little game. Nexon came a long way with it, no questions asked, compared to what it was six years ago, this is a better value proposition. I don't plan to play it anymore, maybe when they completely fill the 1-200 progression with better goals and quests and content to discover, maybe.