Portal: The Flash Version
(I actually played this game long before finding Kongregate, but on Kongregate, it has badges attached to it! So naturally I had to play through it again to get the badges. Fortunately, this didn't take too long.)
The next time someone says to you, "Games of today really don't need their fancy 3D graphics and voice acting; all that really matters is the underlying gameplay concept" (and yes, I know that sounds like something I might say), have them play this game, then have them play the real Portal, and then patiently wait for them to admit that they were wrong. (Since I'm going to be doing a lot of comparison between the original and the Flash version, if I say "the original" or just "Portal", I'm talking about Valve's original product. The Flash version I'll call "TFV". Hopefully this will at least somewhat reduce confusion.)
Portal is the perfect example of a game which pairs an innovative gameplay concept with excellent writing (and the voice acting implementing that writing) in order to make a truly great game. TFV totally lacks that deft writing touch; the writing (which appears in floating text boxes at the top of the screen rather than in a disembodied GLaDOS voice) is functional rather than amusing (compare, for instance, how the two games introduce the concept of player death to the player). And, of course, it's hardly accurate to call the gameplay concept in TFV "innovative" when it's been, ahem, borrowed directly from Portal (which, in turn, got it from another game, but at least they paid for it).
The gameplay in TFV is pretty much identical to that in Portal, although the perspective shift from first person to two-dimensional third person side view naturally affects game play significantly. Because of the limitations of Flash, the simple two-button interface of Portal is absent; instead, the left mouse button alternates colors, and you can use keys to open portals of a specific color. There's also a button to close both your portals, which is quite useful and something I found myself wishing for occasionally when I went back to the real Portal. Most of the elements in Portal (cubes, buttons, turrets, doors, and Aperture Science High-Energy Pellets, though not fizzlers or the Weighted Companion Cube) are present in TFV; TFV also introduces a few new elements: blue plasma fields, which are solid with respect to you and cubes, but which the portal gun can shoot through; red plasma fields, which can also be shot through but result in instant death; crushers, which pretty much do what they say; and electric fields, which zap you when they're on. As you can see, many of these elements are focused on killing you, making TFV a much more deliberately hostile environment than Portal. (Yes, I know that there are crushers in parts of Portal, too, but those are just pieces of machinery designed for other purposes; the crushing is just incidental. The ones in TFV are covered with person-crushing spikes, and are clearly designed for one purpose only: to stab the unwary or slow test subject.) The game is also somewhat glitchy, especially (but not only) when you're carrying boxes around; they can end up in the weirdest places if you're not careful.
The presentation is average. The graphics are OK but nothing special, the sound effects seem somewhat, um, familiar, and the music is OK but not really quite appropriate. The interface is minimal but has pretty much everything you would want an interface to have; the one thing that can be somewhat frustrating is that there's no way to save your progress in the middle of a level (either voluntarily or with some kind of checkpoints), which can get really, really annoying in some of the later, longer levels.
As for the level design -- I will be the first to admit that, for all that I love Portal, the level design is only fair, at least from the perspective of requiring you to think creatively. Although the portal gun opens up all sorts of crazy possibilities, if your goal is just to get through the game, variations on the Fling will be nearly everything that you need. (The challenges, on the other hand, do require you to deploy more of your arsenal.) Unfortunately, TFV doesn't really take advantage of this opening; the levels in TFV are pretty well thought out (and there are more of them than in Portal, which is a plus, although since the levels are limited to the size of the screen in TFV, the total amount of puzzling you have to do is probably the same or possibly even less), but there's not anything which requires a really insightful solution. Because of the greater emphasis on things which can kill you, there's also a greater demand for being able to execute complicated maneuvers quickly and precisely than in Portal; this definitely makes the game harder, but not in a way that I would consider as much fun. I would prefer the game being harder in terms of requiring more thought than more dexterity. There are also a few levels which are just super-frustrating in requiring perfect timing and execution.
Since it might sound like I'm totally ripping on TFV, I should point out that falling short of Portal, which is a great game, is nothing to be ashamed of, and the gameplay concept is still a lot of fun. Overall, TFV is a fun way to spend an hour or two, but after you've played the real thing, it will feel like a pale imitation.