Actually, DC-10s, 767s, A-300s, and L-1011s can, and do land at LaGuardia. However since LGA has mostly short-haul flights, you don't see them very often. Delta currently flies 767s and L-1011s from LGA. American used to have DC-10s from LGA to ORD (and I even took one from LGA to ROC once!). Eastern used to use the L-1011 and A-300 on the Shuttle to BOS. The two restrictions at LGA are runway length (6200 feet or so), and max gross weight for the piers. So "mid-side" planes like these can use LGA if they are light enough (short route, so not as much fuel).
The jumbos which can never use LGA are the 747, A-330, and A-340, not only becuase of weight and runway length restrictions, but becuase the taxiways and gate areas are too small to accomodate them.
Your SubTalk Aviation Meteorologist,
Todd (meteorological consultant for the USAirways Shuttle!)