Cave Story is one of the first games to define the "2D retro platformer with pixel art" trend that has served as the default genre for much of the indie videogame scene for the last several years. Originally released as a freeware title on PC, it's made its way to close to a dozen other platforms over the years and has garnered a reputation as one of the best 2D Metroid-style platform games of the decade. In practice, however, the actual game doesn't especially live up to that reputation - while it's got some good ideas and plays well, forced repetition, an overly convoluted story, short game length and occasionally frustrating controls render it less a game than the classics it apes.
Cave Story's biggest flaw is that it combines a whole lot of gameplay elements together into a big mixing pot, and doesn't take much care to actually make them work cohensively. There's shmup-style weapon upgrade levels that decrease as you take damage. There's Metroid-style missile packs and health upgrades to find. The controls feel very similar to the original Super Mario Bros., floaty and momentum-driven in a slightly imprecise kind of way. There's an RPG-style inventory menu which serves absolutely no purpose in gameplay. And, of course, there's tons and tons and tons of dialogue which serves no purpose other than to drive along the meandering, uninteresting storyline that seems to think it's much more interesting and epic than it actually is.
When I found myself enjoying Cave Story, it wasn't because of any subtle fusion of gameplay elements that just worked well together, it was because there was one individual element that happened to work well in a given scenario. Usually, it's the shooting elements - Cave Story features a decent variety of enemies and weapon types, all of which behave a little differently and according to predictable patterns, making optimal play a matter of juggling your arsenal to adapt to the threats at hand. It feels very SNES-era and recalls titles like Contra, but without the same level of challenge (mostly due to your ever-growing health bar).
The other elements of the game that feel borrowed from Metroid and JRPGs are distinctly less successful. While a lot of people compare Cave Story to Metroid, in truth it has none of the intelligence, finesse and tact of Nintendo's famous game franchise. Whereas Metroid is a game that uses level design to effortlessly guide players towards goals by gradually unlocking new ways to explore the environment, Cave Story simply sends your character platforming back and forth around the levels, sometimes five or even ten times over, in pursuit of collectable. Metroid is a game that lets you figure out how to progress on your own; Cave Story gives you twenty dialogue boxes that eventually end with "go find my five dogs scattered around this level and come back."
Where Cave Story does let you explore on your own, most of it lacks real direction and pacing; there are alternate endings, hidden items etc., but there's no logic to their placement and you'll miss out on things because you didn't pass up or collect the right items previously in the game - unless you're reading a guide as you play, it's more dumb luck than skill whether you can unlock the final stage, or the best ending - did you talk to X NPC at the right time, or backtrack for 30 minutes to find Y secret that unlocks Z door for no clear reason? There are choices to be made and some gameplay consequences to boot, but the lack of coherence behind them robs them of impact.
When the story does take over, Cave Story fails to understand the strength of suggestion and the "show, don't tell" rule. It's not so much that anything about the story or writing is especially bad, but there's simply far too much of it and it's constantly at odds with the fast-paced platforming. Many of the plot details ring completely hollow because there's no emotional depth or complexity to anything going on. "Revelations" that are clearly intended to be shocking or gripping aren't effective in the slightest - at one point halfway through the game, you learn that your character is actually a robot (or android? whatever?), but nothing interesting or meaningful is done with this, and I certainly wasn't given reason to care about my character in the first place. Cave Story has the plot of a Mega Man game spun into 1000+ lines of dialogue and many, many scripted scenes - and it's exactly as enjoyable as that sounds.
Beyond all this, there are a few minor issues dragging Cave Story down. Its rigid adherence to its plot means that many sequences in the game must completed in a very specific order, and if you miss talking to a character you'll have to replay the same section over again as you run back and forth between goals. This isn't helped by the fact that there are relatively few levels in the game, and when combined with both the intended and accidental backtracking required by the gameplay, you'll quickly grow tired of them. I'm certainly not exaggerating when I say that Cave Story would likely be an hour or two long at most if it wasn't for the dialogue, and the fact that it makes you fight and jump through the same sequences over and over (though it's still only about three hours long even with all that). This is occasionally exacerbated by the floaty, awkward platforming, and while the levels are large enough to accommodate it, I never felt like I was truly in control of my character in the same way I am in, say, Super Mario World or Super Metroid, and the similarly floaty camera only adds to the feeling of imprecision.
So, is Cave Story a bad game? No, not really. It's definitely got a few moments of enjoyable gameplay, mostly in the combat and boss battles, and I liked the shmup-style upgrade system that encouraged consistently good play. But it's also a game that feels bloated and overlong, pretentiously full of itself, and simply can't execute its ideas with anywhere near the same level of intelligence that all its inspirations did years ago. Considering it's available free or for fairly low prices on so many platforms, it's not a terrible way to kill a few hours - but when judged next to the genre classics or even other indie titles, it falls very, very short.