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At first glance, business jets and regional jets seem virtually identical - both are small jets, usually low-winged twinjets (unless they're by Dassault, in which case they're trijets) with tail-mounted engines and T-tails.
What differentiates, say, a Legacy 600 or a Falcon 9 or a G650 or a Learjet 75 from a DC-9 or a CRJ-100 or an F-28? Is it merely who operates it?
I would say its not quite who operates but how its operated. A business jet is typically outfitted to carry a small number of people in a very luxurious way while a regional jet is outfitted to carry a lot of people a short distance.
No one even says business jets have to be small or of a select list of airframes, you can order a 747 business jet if you have the cash. Similarly no one says a regional jet has to be small, we are now seeing 747SR's doing short haul flights in Japan.
From a legal standpoint, the difference may be in the operation type as well. Scheduled regional flights here in the US will occur under part 121 while charter business operations will generally occur under part 135 if you own the plane the operation may even occur under part 91. This mainly has differences on inspection intervals for the airframe and some other operational related things. Assuming you can land the plane on the runway and depart again safely these don't limit where you can go.
At a design level, the biggest difference is in design for intensity of utilization. That is, operating cycles per year. An RJ designed as an airliner is intended to operate on average, about 2500 cycles per year. A corporate jet typically operates under 500 cycles per year (although with a charter operator or fractional ownership operator or a corporate shuttle they may run 1000 cycles plus).
The utilization drives the level of robustness required in structural and system design. A purpose built corporate jet put into airline use would have much lower dispatch reliability because it's getting beaten to death (like putting a luxurious but fairly lightly built car into service as a taxi). At the opposite end, you have regional airliners that are sold as business aircraft. These airplanes are overbuilt for their role as corporate airplanes and have a reputation for being pretty bulletproof.
Production wise, the biggest difference is that corporate airplanes are built at a lower rate and are almost always delivered "green" to a completion center selected by the customer to get the interior done (by Supplemental Type Certificate), whereas regional jets get their bus interiors done in-house. There are a few exceptions to this (some Global Express interiors are done in-house by the OEM). And the interiors themselves are built to a completely insane quality standard - I mean, the interiors have to be absolutely perfect when someone has 5-10 million just into the interior.
On customer acceptance on a corporate jet, the customer's acceptance team will be going around the interior while the airplane cruises at altitude making sure all the panel gaps are still straight with the hull pressurized, in addition to making sure everything related to the interior works (like plumbing doesn't freeze etc). The slightest flaw requires rework. God help you if you ding the woodwork...
