Is Walt Disney World crowded in the first weeks of December ?

No the park generally is not very crowded that time of year. You will still have a lot of people but its not as bad as the summer months or the week between christmas and new years. You will not just walk on every ride. I would expect waits of 20 to 40 mins for most rides with the big E ticket like Space Mountain or Soarin being at the higher end. Use the Fast Pass and you will be fine. You will have a great time regardless. Nobody can do christmas like Disney can.