I've looked at several videos of Portal 2 and am interested in buying it. The problem is that I've never played the original Portal, the first game of the series.
Should I play Portal before playing Portal 2?
I think it's definitely worthwhile playing Portal first. I just started Portal 2 and would probably have been a bit lost without having played Portal first. There is almost no introduction to how the portal-gun and other stuff works in Portal 2 and there are a lot of references to the previous game.
It makes Portal 2 an even better game if you have played Portal.
Portal 2 starts out fairly simply. Thus, from a gameplay standpoint, you won't have a hard time figuring out the mechanics of the game.
However, Portal 2 continues the storyline from the first game. There are also many elements in the game that you wouldn't understand/enjoy/love not having played the first game.
Playing Portal 2 without playing Portal can be likened to hearing the punchline, but not the joke.
Furthermore, if you EVER wish to play the first one, it would be ruined by having played the second one first.
Certainly a lot of the story elements will make more sense if you have played the first game, however I don't think it's absolutely necessary.
As for gameplay instructions and explaining how things work, yeah, you'd be much better off playing the first one first. Portal 2 drops you much more suddenly into the thick of things than Portal 1 did.
Those reasons aside, you should get Portal 1 simply because it's a kickass game!
Gameplay: Portal 2 can stand on its own. The first levels of Portal are also in Portal 2, letting people in on the basics. The later levels explore techniques and mechanics that are not present in Portal 2. Thus, playing Portal and especially mods like Portal: Prelude can give you false expectations for Portal 2.
Plot: this is not really true. Wheatley vaguely hints about what happened in Portal, then GLaDOS "gives it away".
So yes, play Portal before Portal 2, if you can.
As already suggested and accepted play the original Portal before starting the sequel is preparatory to have even a greater experience with Portal 2.
But suppose that you are so excited because of all wonderful reviews and nice comments you read around, and you really want to start immediately Portal 2, while the game is installing you can spend 15 minutes doing the following things:
You should definitely play the first game.
Not only because it's important to understand the story, but also because its just as excellent a game as Portal 2 is.
You can play Portal 2 without having played Portal first as every game mechanic is presented in Portal 2. However, you'll spoil the story of the first one doing that as it is a direct sequel (and there are many references to the first one, in the story and in some puzzle).
I think it would be a good idea to play the first one, because once you have finished playing Portal 2, you'll want to play it.
Any other things you might think of.
There are other reasons for playing Portal 1 before 2 that are not really detailed in the other answers.
And as others have stated, Portal 2 is a sequel that follows on from events in Portal 1, but in a more story-driven manner. There is a sense that you are being thrown into the game ready to handle the faster pace of Portal 2 because you went through Portal 1.
I happened to play Portal 2 before I played Portal 1 (mostly because Steam had 2 on sale at the time).
While playing 2, I didn't really feel that lost. Most of the mechanics shared between the two games (portals, buttons, cubes, moving platforms, turrets) have similar “tutorial” levels. Story-wise, the two main robot characters in 2 will tell you how 1 ended. Which would be a disadvantage if you don't like spoilers.
There were a few references that didn't make sense until after I played 1. Most notably, the appearance of a particular prop in 2's final cutscene.
Having thoroughly enjoyed 2, I went on to play 1. But it was kind of a letdown, having only about a quarter of the play time, and missing the lovable characters of Wheatley and Cave Johnson. I probably would have appreciated 1 more if I hadn't played 2 beforehand.
If you want the full experience of portal 2, I recommend to play the Half-Life -series first and than the Portal-series. Because there are things in the story of Portal that refer to the story of Half-Life.
