Why is pago weird? I don't understand the Frankenstein part either? It uses a vocabulary that has a good bit in common with the romance languages. Not sure what else.
The really weird letter in Esperanto is ĥ. It uses a sound that isn't very common in many languages so it's hard for a lot of speakers to pronounce. (It's like a "ch" sound in German.) Typographically it looks weird. There has been a general trend towards replacing it with a "k".