You could get away with skipping HL1. I'd actually recommend skipping the add-on packs for it (Opposing Force & Blue Shift).
No no no.
As others have said do not skip HL1 or the expansions. HL1 is still the greatest FPS ever and I'd rather play it over every FPS released since. I played through HL1, Opposing Force and Blue Shift a year or two ago and they are still brilliant games (Blue Shift less so)
Opposing Force is probably the best game expansions I have ever played and could have been released as a stand alone title in it's own right rather than an expansion to HL1