The rest of you have stated it already so well. First of all, I will say that it is a fairly well written letter, just not realistic or fair.
I agree that either there was a mis-understanding or the travel agent did a poor job at explaining the First Class voucher. I suppose it is because I typically do not fly this airline, but I do fly extensively for work and pleasure, in all classes on the plane, and I have never heard of a First Class voucher.

As someone else said, if you wanted to be gauanteed seats in First Class, you should have paid the money. I would be willing to bet that First Class was full, as it often is, on most or all of your flights. I would further be willing to bet that all of those people either paid the extra cash to sit there, or were frequent fliers of the airline who got a free bump. Their free bump will come before yours, sorry. They have shown a history of supporting that airline and providing it with profits. You have said that this is your first flight with them.

Anyways, you did not mention if your luggage has since been found, but I would guess it has. I HATE when an airline loses my luggage, but I usually have it back hours later or at most within 12-24 hours.