I fly out of ABE all of the time (I'm 20 minutes from the airport). For Disney, I've flown USAir, Delta, and United in order of preference. The keys for me are the departing times, interconnecting airports, and plane sizes:
USAir - We've got a flight booked for September that leaves at 9:00, I'll get to the airport around 8 and have plenty of time. Advantage - I can sleep a little later. The connecting airport is Charlotte - the advantage here is it's a small enough airport that delays are rare and getting from gate to gate isn't that bad. Plus the food court is pretty good. We arrive in Orlando around 1:00, so we have a good part of the afternoon and evening to spend in the parks. On the return flight, we leave at 5:00, so we've got additionall time in the parks that day too. We'll arrive in Allentown before 10:00.
Delta - Our last trip was on Delta. That flight left at 6:30, so it was an early morning (and I'm not a morning person), with a longer wait in ATL. Atlanta is not a fun airport, and is notorious for flight delays. I think we arrived in Orlando around 1:00, so we did have the rest of the day in the parks. The return flight left at 6:30 (even more time in the parks), but didn't arrive at ABE until around 11:30. This is past my wife's bedtime so it was a struggle for her.
United - United has a morning (8:00?) flight through BWI. The plane from ABE to BWI is a tiny commuter (Saab) that feels claustrophobic, doesn't have good cabin storage (you end up gate checking carry ons.), and is noisy. BWI uses a common connecting gate/waiting area, so it gets crowded. Food court is so-so. My wife refuses to use the commuter plane anymore, so I doubt I'll be booking on United anymore.
That leaves Allegiant. They fly into Stanford, so you lose ME. They also have a limited flight schedute, leaving midweek and later in the day (around 4:00), but they do fly direct. I've never used them.
Hope this helps.