you don't need to play hl1 to play hl2 since neither of them has much of any story and hl1 has almost nothing to do with hl2 (and no characters at all)
Not true. So, who is Barney Calhoun? What's his significance? How did the Combine get in to our world? Through a portal, opened in a Black Mesa experiment... which Gordon Freeman (main character of HL and HL2) unwittingly took part in. Barney Calhoun was a security guard at Black Mesa, who helped Gordon behind the scenes. Adrian Shephard from HL: Opposing Force is a US Marine, whose unit was originally tasked to suppress the alien invasion, but whose directives then become to erase all traces of the incident, including any witnesses (he doesn't follow those orders, and neither does the squad of survivors he eventually joins). I have not seen him in the HL2 series yet. Barney, however, makes several appearances and assists you in HL2. There are numerous inside jokes and references which you would not realize came from the original HL.
In addition, Aperture Science (from Portal and Portal 2 with the malevolent AI GLaDOS), if :v: had decided to pursue a link between FS and Descent further, Portal would be Descent) is also mentioned, and will most likely play a significant part of HL2: Ep 3 (HL2 part 4), as one of their devices is loaded on one of their ships, the Borealis that disappeared, along with a significant portion of it's dock, and has been found in HL2:Ep2 and 'must be destroyed', according to Alyxx's father, Eli, but should be used as a weapon against the enemy according to Dr. Kleiner, one of the White Forest colleagues you meet.
Pretty much, what you just said is like saying you can also play FS2 without FS. The questions about the storyline of HL are answered and filled in in HL2, however if you didn't play HL, you will be left wondering about certain references. ;)
EDIT: quote from wikipedia synopsis of HL2 (#1, not Ep1, which is #2):
Half-Life 2 is a work of science fiction which presents a dystopian alternate history of Earth in which the resources of the planet, seemingly including the human species itself are being harvested by an oppressive multidimensional empire, known as the Combine. The game is set around the fictitious City 17, somewhere in Eastern Europe, almost 20 years after the events of Half-Life. During Half-Life, the scientists at the game's Black Mesa Research Facility cause an interdimensional instability, known in the series as a resonance cascade, which Gordon tries to resolve. However, by killing the overlord of the attacking "border-world", Xen, Gordon unwittingly widens the dimensional rift, which leads to disasters on Earth.
Some time after the ending of Half-Life, this instability attracts the attention of the Combine, and they invade. Humanity surrenders at the conclusion of the resulting Seven Hour War. City 17 becomes the home of the gigantic Combine Citadel, and Dr. Wallace Breen, who had managed "the Earth's surrender," is appointed there as Administrator to supervise the survivors on behalf of the Combine. Unable to procreate because of the Combine suppression field, humanity matures, and no children remain. The Combine implements a brutal police state of Civil Protection officers and Overwatch soldiers, and the underground Lambda Resistance forms.
http://en.wikipedia.org/wiki/Half-Life_(video_game)
http://en.wikipedia.org/wiki/Half-Life:_Blue_Shift
http://en.wikipedia.org/wiki/Half-Life:_Opposing_Force
http://en.wikipedia.org/wiki/Half-Life:_Decay (PS2 only, but some story line exists here that you run into in Blue Shift and Opposing Force)
http://en.wikipedia.org/wiki/Half-Life_2 (#1, ep 1 and 2 are parts 2 and 3 of HL2)
http://en.wikipedia.org/wiki/Half-Life_2:_Episode_One
http://en.wikipedia.org/wiki/Half-Life_2:_Episode_Two
http://en.wikipedia.org/wiki/Portal_(video_game)
http://en.wikipedia.org/wiki/Portal_2