i don*t know UA's economy plus but usually it's something between normal coach and Business class. VS Virgin has it also on some long haul flights. Usually Airlines who have it charge the so called Full Flexible economy fare which is the IATA One Year fare. ( about 15-20 Percent lowern than Business but still more expensive than budget coach fares, APEX, Excursion and others)
Economy Plus is Economy Class with more legroom. It is available to those who bough full-fare economy tickets or if you are a preferred level frequent flyer. Currently, it is available on all 727's and 757's. It will be on all of United's domestic planes very soon.