I've been there at many different times of year -- and if you have been to WDW then there are a few comparisons you should be aware of:
It is NEVER as crowded at DRP or DSP as it is at WDW at peak. Their attendance is highly dependent on the European school schedule - and weekends should be avoided year round if possible. Like the US parks, you need to get there at least a half hour before opening. Make sure you have already purchased your tickets and have them in hand (either in Paris when you arrive at various locations, or from Disneylandparis.com mailorder via internet.) The lines are long and unwieldy.
Like MK, they let you into the park about 15 minutes before opening. Head directly to the Space Mountain rope drop area - ride as soon as they open and you will have no line, and most likely no line to re-ride. WIthin a half hour there will be a half hour wait. Then head directly across the park to Frontierland and ride Big Thunder Mountain. From that point on you can wander. Two adults, on a schoolday, nonpeak, can do the ENTIRE park by 3 pm. THere are fewer attractions and the actual park is much smaller than the MK.
Add a park hopper option to your ticket and you can head over to Disney Studios afterwards.
By all means - avoid the beginning of school holiday periods, and weekends. You will find a huge difference between crowds at WDW and crowds at DRP. Crowds in WDW, even when at peak, stand patiently (well, not always patiently) but at least stand in line and wait their turn. The crowds at DRP are mostly European-type crowds...that is LINES ARE NOT EXISTANT until you actually get into a single file queu line near the loading docks - people push, work their way past you, cut in front, and sometimes ignore lines altogether. Lines in France are merely an inconvenience, and the "priveleged" will push ahead of you no matter what. I don't mean this to be derogatory, since I lived in Paris for many years, but there literally is NO concept of a single file line in which you wait your turn.
This will also apply to counter service restaurants, the entrance queues, bathrooms and everywhere else. There is no other way to minimize this but to avoid peak times ( weekends, and school holidays).
You should also be aware that while American's at WDW will head in all directions at MK, EUropeans will primarily start in Frontiertown and work their way around clockwise. That is, except for all the teens and people in the know who head directly to Space Mountain. There is no second big attraction to draw the crowd at opening, so Space Mountain is it.
DRP also has some odd riding patterns -- people actually "Ride" the train -- that is, they treat it as a major attraction and go all the way around in the circle and don't get off at any of the stops. Unlike MK, you can not use the train to make a quicker time-saving trip from, say, Discoveryland to Frontierland. You won't be able to get on the train at all at busy times of day. For some reason, they have no problem waiting an hour at the mainstreet station to ride the train around in the circle.
The Castle area, unlike MK, does not serve as the primary "meeting point" for most people - as we know from MK, most AMericans meet somewhere in front of the castle, or near the Walt statue. At DRP, the meeting places tend to be the restaurants and arcade areas on Main Street. Avoid those.
Do a google search for the school holidays for France, Germany, and Italy. Those will be your primary guidelines. Also - be on the lookout for specials announcements on the disneyland resort paris website -- if they are running a special like "get two free nights at a resort hotel with the purchase of two nights" the parks will be PACKED...those kinds of bargain incentives drive the crowds more than holidays at times.
Ive been there at many different times of year, and have never had a problem if you go midweek and avoid July, August, and the last two weeks in December.
BY the way, the Disney parks interchange their holiday decorations - you will recognize the holiday decorations from previous years at WDW. Last year, they had some original decorations on Main Street, but the previous year, they used Epcot's decorations from the previous year.
