Hmmm... To me it really doesn't seem all that necessary...
I know Jason's a bit more into Half-Life, maybe he'll disagree
Some of the supporting characters from HL1 make a return (and will hint that you should remember them from Black Mesa, the complex in which HL1 took place), but in the original they had absolutely no personality... They were just there to help you open doors and fight monsters.
Your "old boss" from Black Mesa makes a "return"... If you aren't familiar with the original, you may think you're missing out on something... But you're not... He wasn't even in the first one.
The circumstances in HL2 have changed drastically and in such a random way, it seems like it'd stand on it's own just as well...
The only real important plot details I can think of is that various dimensional rifts were opened up, and at the very end of HL1, Gordon is detained by a mysterious suit (everybody calls the G-Man)...