An interesting discussion. Let me offer a simple solution - Legolas had better eyesight than anyone else in the Company. A Balrog is wrapped in shadows and is not easy to be recognised at once.
We can be pretty sure that Boromir and younger hobbits knew virtually nothing about Balrogs, while Frodo and Gimly potentially could have heard a little (I mean the Barlog as a kind of creature, not Durin's Bane before it was identified). Aragorn seems likely to have come across some information about Balrogs' role in the first age but could hardly expect to meet one; Olorin, as far as I concerned, had never met one in the flesh. For Legolas Balrogs were the living memory of his race. All of them were aquainted with at least one person having a first hand Balrog experience (Glorfindel), and Aragorn should have known him since he was a child. However the perception should have been quite different. Imagine, you walk through a London suburbian park and see an elk running towards you. You probably are going to be taken by surprise and feel fear. That's what Legolas feels. And now imagine meeting a dinosaur in a simillar settings. In this case you probably would struggle to come to terms with your own mind. This could be a reason why Aragorn didn't name the thing in Lorien. He probably thought: it was too difficult to believe and they would think he lied.
What is unclear for me is that dwarfs failed to recognise a Balrog in Durin's Bane. But yes, probably they forgot those events...