Here are three ways of splitting the letters of the English alphabet into two groups:
1) ABDOPQR; CEFGHIJKLMNSTUVWXYZ
2) BCDGJOPQRSU; AEFHIKLMNTVWXYZ
3) DFHIJLNOPRTUVXYZ; ABCEGKMQSW
In each case, the first collection of letters can be described simply using a one-word 'catchphrase' while the second collection is simply all those letters not in the first collection. First you need to find all the three catchphrases (this should be the only challenging part). In each case, determine how many distinct letters of the catchphrase are in the first and how many in the second collection of letters. You'll then have three pairs of numbers, and your final task is to find which of these three pairs is the odd one out.
What I really hope to achieve in this question is to get a collection of interesting alphabet-splitting puzzles, so that they're not all spread around the site in many different questions. The bit about catchphrases is just to unify this into a single puzzle so that I don't get people telling me I should split this into 3 distinct questions. After this has been solved (which shouldn't take long), I'll throw open the question to editing and we can incorporate all the interesting ways of splitting the alphabet that anyone can think of, and perhaps change or remove the catchphrase and odd-one-out parts of the question. When editing a sequence into the question, please edit the answer into warspyking's now-accepted answer.
4) ABCDEGJLMOQTUZ; FHIKNPRSVWXY
5) EITSANHURDMWG; VLFBKOPJXCZYQ
6) ABCIJOPQRSTUWY; DEFGHKLMNOVZ
7) COPSUVWXZ; ABDEFGHIJKLMNQRTY
8) AHIMOTUVWXY; BCDEFGJKLNPQRSZ
9) ΑΒΕΗΙΚΜΝΟΡΤΧΥΖ; CDFGJLQRSUVW
10) AEFHILMNORSX; BCDGJKPQTUVWYZ
11) HJKMNUVWXY; ABCDEFGILOPQRSTZ
12) BDEFHIKLMNPRTUVWXYZ; ACGJOQS
13) EFPTY; ABCDGHIJKLMNOQRSUVWXZ
14) BCFHIKNOPSUVWY; ADEGJLMQRTXZ
If you think this is a silly way of posing a question, please tell me and give me a chance to improve it before hitting the downvote button. Let's be proactive here! :-)
Edit: I just discovered that more or less exactly the same idea appears in Schott's Miscellany. His list is entitled 'Letter Traits' and is on page 76.