The Paper Mario games often feature RPG elements and contain a humorous storyline which takes a departure from the usual 'Princess Peach is kidnapped by Bowser' story. However, that's the story in this game and it doesn't really get more complex than that. There's a bit of humour in here but it is very light. The RPG elements are basically gone too. So what you end up is a huge step back for the series.
There's a sticker festival which Bowser gatecrashes to get his hands on the Sticker Star. It breaks into 5 fragments, and Bowser makes off with the Princess. It's up to Mario to retrieve the pieces and the Princess.
Mario must make his way through the cliched themed worlds; grass, forest, desert, ice and fire. In each level, touching an enemy initiates a turn-based battle. You use stickers to battle which are one-use items. Winning the battle only rewards you with coins and possibly stickers – there are no experience points. This means the majority of the battles are just there to pad the game out, since you can just skip the battles with no repercussions.
Once you have selected an attack, you can time a button press to determine the attack's success or extra damage (depending on the weapon). For example, the standard jump move will deal extra damage with a correctly timed press. The hammer has a wider range, so will do nominal damage if you fail to press a button, do decent damage if you press too early, or do lots of damage with a perfectly timed press. When enemies attack, you can reduce damage with a correctly timed block.
You can normally only choose one sticker per turn, but you can choose to spend 3 coins on a fruit machine spinner for chance to use 2 or 3 stickers in that turn. You can spend a further 15 coins to lock 2 reels in place to guarantee your success, and spend more money to slow it down. This spinner mechanic is vital during the boss battles and often useful during normal battles.
There are plenty of stickers to peel off the environment, stickers to be found in blocks, or you can purchase stickers at the shop. The game is fairly generous with coins and stickers, so you should always have a decent supply of them; although a large part of the game is resource management. Additionally, the stickers respawn if you quit the level and replay it, meaning you can farm the stickers.
There are puzzle elements to the levels too. There are certain objects that may look suspicious, or be peeling away from the background. Mario can enter the 'paperize' mode and rip out these objects and place them in his scrapbook. He can then place them at another location (or sometimes it's just a case of replacing them at a different orientation) to progress.
You may find real world objects simply known as 'Things'. Once you have found a 'Thing', it will be able to be purchased again at a special shop. These can be turned into stickers which provide massive damage in battle, or be used to solve puzzles within the levels. Some of these puzzles are fairly obvious like using the bowling ball to knock over the bowling pins. But you will also use a vacuum cleaner to suck up a tornado or use a fridge to freeze lava. It's moments like that when you will end up reaching for a walk-through; but this shouldn't be the case in Mario games. 'Things' are often vital to take down the bosses, but there will be no hint prior to facing them, meaning you will have to do some tedious backtracking to get the stickers then go back to fight the boss again. Luckily, I always had discovered the 'Thing' so was able to go to the shop to obtain the sticker. If you don't have it, you wouldn't know which level you need to replay to find it without consulting a walk-through. You would expect the level select screen to show you if you have found the 'Thing' within; but it does not.
Some levels have multiple stars in them, but the alternative path through the level will be hidden. This is fine if it is a secret, but these stars are mandatory, which is just ridiculous. I was getting really frustrated with this game on several occasions as I couldn't progress to the next level.
There's the occasional use of platforming, but I often found this to be poor due to poor depth perception. I was constantly missing platforms and having to retry sections.
In previous Paper Mario games, there will be support characters that help you in battle, but the only character that assists you in some way is a sentient crown called Kersti. She does say she will give you hints when you press the Left shoulder button, but she isn't helpful at all.
There's plenty of tedious design choices within the game. When you finish a level, it will autosave but your health won't be restored. You can then select Decalburg on the map and restore your health for free, return to the map, then go to your next level. This is why I think it should just auto-heal if it gives you the option for healing for free anyway. Secondly, there's a sticker museum in Decalburg but you have to go through a door, down the stairs, through another door, walk up a different set of stairs and down the appropriate corridor to place each sticker in turn. I don't get why they couldn't just let you go through one door and ask you which stickers you would like to hand over. The process is a complete chore.
Sticker Star is a huge step back. This game has lost a lot of the series' charm and has become really shallow and tedious. It's not a complete disaster, but it's the worst in the series and pales in comparison to the likes of "The Thousand-Year Door".