People assume he's going to walk in and run rings around everyone, he has to beat Rosberg first, who made the 7 times world F1 champion look decidedly ordinary for a couple seasons.
They also don't have a recent history of producing race winning cars. This is risky move on Hamilton's part and we'll only know if it was sensible or not in about 2-3 years time as I have a feeling this move has been made with the 2014 rule changes in mind.
You also have to consider that if Mercedes don't win they won't continue to play - they are first and foremost a car manufacturer that also happens to race. If the results aren't there the "Board" will pull the plug in a 5 minute meeting.
Ferrari, McLaren and Williams are Formula 1 teams, two of them happen to make cars also. They will be there year in year out forevermore.
Bold move Lewis.