When I first heard about Portal, Portal 2 was available and popular, so I bought it. I liked Portal 2 a lot because of its puzzles mostly, but the story and jokes are nice too.
I'm wondering whether I should buy and play Portal now, so what does Portal have that Portal 2 doesn't?
Mechanically, Portal has almost nothing that isn't in Portal 2. 2 was built upon the success of 1, expanding the story and gameplay. Where 1 takes about 1-2 hours to complete, the single player story in 2 has up to 10 hours of gameplay on a single playthrough.
In terms of design, 2 didn't deviate that much from 1, instead adding more of the same. The puzzles in 2 add new mechanical elemants (goo, for example) that weren't present in 1. As MrLemon pointed out, the energy ball was removed in Portal 2, but was replaced by the lasers which exhibit mostly the same behaviour in the puzzles. All that said, if you played and enjoyed 2, 1 will be mostly more of the same thing (which is a good thing).
What Portal 1 does have that 2 doesn't is the particular configuration of puzzles (such as the boss battle). More importantly, it has its own storyline and develops the characters, which leads directly into the next game. If you enjoyed the humour and characters, the first game has some of this and playing it will lead to a better understanding of the sequel.
An important point is that Portal is a short game and is available cheaply, so you're not at risk of spending a ton of money or time in trying it out.
Imagine it's kind of like Sudoku or other puzzle games: You have the same set of rules but different "problems" to solve.
If you enjoyed Portal 2 go and grab the first part as well. Its fairly cheap right now ($2,49 where I am) so theres really not much to lose from just getting it
