|The "Morph Ball"; once acquired,|
Samus can use the bottom-left path...
A good example present in every game in the series is the "Morph Ball" item; with it, Samus can reduce her body size to a ball roughly half of her normal size, allowing her to enter various forms of tunnels and narrow passages, which in turn effectively "unlocks" more of the game world for the player to explore.
|...and has thus become a staple of the|
series, appearing in most instalments.
A "proper" sense of achievement is hard to find in most modern games, with developers often falling into the trap of making their product either overly sequential (thus effectively eliminating the illusion of choice and the effect said choices have on the player's experience) or downplaying on the player's decisions (most often by burying the player in a multitude of secondary variables and inconsequential choices); therefore losing any sense of achievement altogether.
|An indie title using the Metroidvania|
model, Iconoclasts is worth a look.
A good remedy for the aforementioned problem is the "Metroidvania" model (a portmanteau of the "Metroid" and "Castlevania" series' titles, which are of the earliest examples of this particular style) - in this model, the player initially has limited exploration venues, which gradually broaden with the acquisition of in-game techniques or items. Under this model, the player can effectively pace his experience; at most points they can attempt the next available challenge or hunt for upgrades to their character, thus lowering the actual difficulty of any subsequent content they experience.
|Though not platform games, recent|
Batman games exhibit this model.
It goes without saying that, while optimal, this model does not work for all genres; in particular (but not limited to), first person shooters - which often rely on a pre-determined pace to convey their experience - are the least likely candidates for this model. Adventure games, which give weight on puzzle-solving and story telling, employ a similar structure; they are, however, sequential by design and linear by nature.
Ultimately, it is left to the individual designer to assess whether their project can benefit from this model of level design; it is worth remembering, however, that it offers a sense of progression that can be used to enhance the audience's experience by rewarding progression and pacing - something that modern releases often lack.