I think the guy they got to play Dean in the flashbacks really did a good job of looking like Jensen... but I think he was trying to hard to pull off the kind of personality that Dean has. It took a very complicated person and made him kind of one dimensional.
Exactly. He looked and sounded like Dean, but he failed to successfully nail that which makes Dean Dean - his personality. Nothing against the guy, of course. I just don't think he played the part all that well.

Although I also enjoyed Dean's line about this whistle... "The whistle makes me their god!" but the episode itself was a little disappointing.
The whistle line was indeed great, but as you said, the episode as a whole was disappointing.