As with any crowds a lot depends on the individual's personal tolerance. You will see some report that crowds are so bad they will never do that time again, and others for the same year report that it is fine. Have done Easter time a dozen times over the years, including when it is in March, late in April, and in between. The two weeks are most crowded when Easter falls latter part of March or early April, when there is a risk of park closure at MK during the day because it has reached capacity, but even when Easter falls other times, the crowds still are the second largest of the year behind only Christmas week. Standby lines for popular rides typically go over 2 hours, some ike Soarin and Test Track can approach 3 hours, and can be that way for much of the day after the first hour or two after opening. Even something like Swiss Family Treehouse can approach an hour line mid-morning to afternoon. Waterparks can fill to capacity by 11 or 11:30, and then open up again a few hours later.
Street crowds can be very heavy at the parks, particularly MK and AK where you need to allow significantly more time for getting from any point A to any point B than you would usually allow for during other times of the year, including the summer months. And that problem will likely be matched at Studios next year with a signifcant portion of the park closed due to construciton of new lands. Guests start seeking ideal spots for the MK parade as early as 1 1/2 hours before the event.
Fastpass's are a necessity for any popular rides except that you may be able to catch one or two with a fairly short line if you arrive first thing in the morning, and be aware that Disney often for that time of year opens all the parks early even to offsite guests. Sometimes standby lines for a popular ride can shrink to under an hour and even to a half hour for a short time during a parade. Likewise during the last hour the park is open when open late (MK typically goes to midnight or even 1 a.m.) As far as planning park visits, we have found AK's and Epcot's lighter crowd days are the day before Easter and Easter, although "lighter" is a relative term meaning it is just not quite as crowded as other days. Easter day is more than extreme at MK because so many go there expecting to see something on that day that they cannot see other days of the week; they are misinformed because nothing goes on at MK on Easter that you cannot do other days of the week. For some reason the day after Easter is also usually close to the same extreme, and of course everyday is very crowded at MK.
Dinner reservations for park restaurants and any character meals need to be made well in advance (as soon as you can start reserving under the 180 day + 10 rule for the more popular restaurants). Another increase to be aware of is that menu prices at the restaurants can go higher during that holiday time.
