[Jumping on the bandwagon...]
1. Allen never went to school, but was taught to read and write by Mana very early on. Books were hard to come by, but he read whatever he could find, and picked up a lot during his travels with Mana and with Cross. Because of this, though, he knows a lot of random information while lacking some things that are part of a general education.
2. Allen speaks English, some French and German, fairly good Hindi, and very few phrases in Japanese (mostly insults, picked up from Kanda.)
3. Allen would really like cats if they wouldn't keep trying to eat Timcanpy.
4. Allen sometimes wears hoods and caps to hide his hair, but would never consider dyeing it- it's a reminder.
5. Allen has a love/hate relationship with Cross. That is, he loved it when Cross was missing, and hates having to deal with him again.
6. Despite this, Allen really does respect his Master, but don't expect him to say a word about it.
7. Allen loves to travel and meet new people, so he's finding DDD to be fascinating (especially since he doesn't have to worry that the people he meets here are Akuma.)
8. Timcanpy is Allen's best friend, and he's more than a little annoyed that the golem is fickle and likes Cross more. :\
9. Allen believes in God and Heaven, despite never really participating in organized religion. When you see the souls of Akuma, it's hard not to.
10. Allen sometimes wonders if his birth parents are still alive, and secretly looks out for anyone with a vague resemblance whenever he's traveling in Britain.
11. Allen would probably still win at cards even if he didn't cheat, but he's too proud of his perfect win record to try.
12. Allen eats so much not only to satisfy his Innocence-enhanced metabolism, but because he still isn't quite used to knowing that another meal will be coming soon.
13. On a related note, when Allen says he could die of hunger, he isn't joking around. Once when he was younger, he had to go without food for a week and nearly fell into a coma.
14. Allen doesn't keep listening to Cross because he 'has no spine' like Ichigo claims, but because years have taught him that however strong-willed you are, Cross is stronger.
15. Allen also wonders what poor soul Cross used as his servant before they met, and envies him or her for escaping.
16. Allen likes nearly everyone he meets, but he's still getting used to the fact that people like him back.
17. Allen's favorite food is mitarashi dango, something he first had when Cross met with a refugee from Japan. That was the only time he ever had it until he came to the Black Order.
18. If he were ever given time to think about it, Allen might have a crush on Lenalee. However, things are always far too hectic (and fear of Komui would keep him from admitting it, anyway.)
19. Despite being grateful to be an Exorcist and to be able to protect people, Allen still sometimes wishes his arm looked normal like everyone else.
20. Allen is far more scared of his Master than of the Earl.
This is mostly headcanon, with some elaboration on actual canon facts thrown in.