1. Get there early. My visit was on a weekday and we arrived at 9:15 for a 10 A.M. opening to find significant lines already outside the gates. Also note that Disneyland Paris doesn't do a rope drop (at least the Studios park didn't), instead allowing guests to go directly to an attraction and line up for it even though nothing opened until the official opening time.
2. Unless you really want to do both parks, just do Disneyland Park. It is a full day park by itself, and Walt Disney Studios would be another 2/3 to a full day if you were to see everything there. I tried doing both, and it wasn't worth it since the Studios park doesn't really have any headliner attractions that aren't clones. We did only two attractions (Crush's Coaster and CineMagique) before switching parks.
3. Expect crowds and have a plan. On my visit, we found mid-day waits of 45 to 90 minutes for many attractions. Despite this, we were able to do 15 attractions (plus a couple re-rides) during our 11 hours at Disneyland Park. As far as what to do, it is really up to the individual, but I would suggest the following as not to be missed:
-Big Thunder Mountain (FP)
-Buzz Lightyear Laser Blast (FP)
-Le Chateau de la Belle au Bois Dormant
-Pirates of the Caribbean
-Space Mountain: Mission 2 (FP)
-Star Tours (FP)
The attractions marked FP above are Fastpass attractions, and I suggest using it if possible (especially for Big Thunder Mountain...that ride was at least a 75 minute wait until about an hour before closing). Fastpass is also available on Indiana Jones et le Temple du Peril and Peter Pan's Flight. Note that Fastpass works a little differently in Paris as the return windows are only a half hour instead of an hour and the return windows are enforced fairly strictly.
A few more notes about attractions: First, it is worth doing a little research to find out which attractions are duplicates and can be skipped and which have the same name and theme but are unique attractions. With the exception of Buzz Lightyear, everything on the must ride list is unique to Disneyland Paris in some way, but some of the other attractions (such as the Fantasyland dark rides) are very similar to their U.S. counterparts other than being in French. Second, Disneyland Paris has a number of walkthrough attractions that don't exist at the other Disney parks. Many of these are very well done and are an excellent way to kill some time if a Fastpass return window is coming up. Finally, be aware that some of the attractions at Disneyland Paris are actual thrill rides unlike their US counterparts. For example, their Space Mountain is a fairly intense roller coaster with a launch and several inversions.
Although I recommend just doing Disneyland Park, if you decide to do Walt Disney Studios Park as well I'd recommend going there first and doing Crush's Coaster, then getting a Fastpass for either Rock 'n' Roller Coaster or the Twilight Zone Tower of Terror and riding the other while you wait. Once you're done with those three, get to Disneyland as nothing else at the Studios park is worth it if you only have one day.
4. If you have flexibility to choose what day to visit and there is rain predicted on one of the days, go that day. On my visit, it started raining around 6 P.M. and the crowds disappeared. Everything dropped to below a 30 minute wait, and a majority of rides were 15 minutes or less. For example, Phantom Manor was 30 minutes when we rode around 1 P.M. We went back around 9 P.M. and it was a walk-on. With the exception of the dark rides, pretty much everything experienced a 70% wait time decrease by 8 P.M. If you can't choose a day or there is no rain predicted, just pick a weekday and hope for the best.
Disneyland Paris overall is fairly decent. I do prefer the Disneyland Resort in California (the only other Disney location I've visited), but the parks are still fun and unique. The parks don't have any of the more modern rides found in the US, but they do have a number of attractions that aren't found in any other Disney park in the world.
If you want to do just one of the parks, I recommend the Disneyland Park. This includes some of the best rides: Big Thunder Mountain, Space Mountain, Phantom Manor, Pirates of the Caribbean etc.
However, Walt Disney Studios does include the amazing Crush's Coaster which is worth a visit. The queues can be outrageously long though (we waited two hours).