http://members.aol.com/gulfhigh2/words17.html
Ex.'s:
BELLE-FAMILLE is the longest word consisting only of letters in the first half of the alphabet. PROTOTYPONS is the longest word consisting only of letters in the second half of the alphabet.

The longest words consisting only of vowels are YOUYOU and AYEAYE.

Here's one that reminded me of our Most Beautiful Word thread:
The longest words with horizontal symmetry are COCCIDIE and DECOCHEE. The longest words consisting only of letters with vertical symmetry are TUYAUTAIT and VOUVOYAIT.