There's nothing more disappointing than non-existent fireworks.
Chris Sims recently finished up a series of posts on Critical Hits about dealing with solo creatures. One of the practices he encourages is letting people know ahead of time that there's a solo coming to help build up tension. This should make things more fun. Read his post for all his points, its a good one.
However, I'm currently running a deep points of life campaign. Its a pretty dark world where life s less than great. Sometimes, if the PCs aren't careful, they can stumble into some pretty nasty surprises completely blind. So recently the PCs accidentally angered an adult silver dragon. The fight that followed was pretty one sided. The PCs wiped the floor with him. The warden and warlord kept the guy flanked in two zones, and there was grab grass present. Each turn he could either try and reposition himself. Silver Dragons are brutes so his to-hit and defenses weren't all that great. He never got out of the zone, and was prone at the end of every round.
The odd thing is, this fight was a success. The PCs loved it. This was not their first 4e dragon and each time before they had a much tougher times, including one that could have been a TPK, where they decided the best way to fight a blue dragon was on a boat. So this time, even though it was not much of a fight, they loved it. They felt powerful, and that was fun. Its been a long road to mid paragon, and it was great for them to just be awesome. I stopped the fight at one quarter hit points none the less.
So sometimes I think its ok to hear fireworks but not see them. I'll do my best to make sure it does not happen twice in a row however.
I also played with the Oblivion Moss from the MM3. It was good fun. I took the suggestion to make the players roll for their dopplegangers. This was frustrating experience for them. I found the more I described how the Mindmasters were scanning their minds and how the other mosses were beginning to look more and more detailed the more interesting it was for the players. They don't have the book yet and they began frantically speculating what would happen if someone died.
That gives me an idea for a monster. . .