I think IAD will still be a hub for United but not as large as what it has been until now. They'll probably eliminate international flights except IAD-LHR and make of IAD a domestic hub on the East Coast like RDU used to be for American. They may keep, however, their direct flights from IAD to these points in the West Coast:SAN, LAX, SFO and SEA.
Yes, sure, it will still be an international gateway but mostly for foreign carriers. Other Star members such as Lufthansa and Varig will add flights or take over United's international flights to/from IAD. I see IAD as a connecting point in the Star Alliance network, not just United anymore, one would fly for example Lufthansa from FRA to IAD and then connect on United or United Express to an other domestic point on the East Coast. I don't know if Austrian flies to IAD, but as a Star Alliance member, it definitely could.
You won't see a United 747-400 anymore in IAD, the only US airports where you will still see those are LAX and SFO, and maybe ORD only if United continues to fly nonstop from ORD to the Pacific Rim.