(Very) partial answer
The message (which may just be the beginning of the puzzle) is:
Did you notice the __o_i__
But I can't find any dictionary word that satisfies the final paragraph as it stands...
The first is in cod but not anchovy
D
The second is in fire and also in inferno
F, I, R, or E
The third is in aikido but not in jujitsu
A, K, D, or O
The fourth is in hey and also in hiya
H or Y
The fifth is in navigator but not in traveller
N, I, G or O
The sixth is in obtuse and also acute
U or E
The seventh is in optician but not in optic
A or N
The eighth is in song and also in carol
O
The ninth is in entrance but not in amaze
N, T, R, or C
The tenth is in email and also listserv
E, I, or L
The eleventh is in car but not in axle
C or R
The twelfth is in cider and also in beer
E or R
The thirteenth is in star-roof but not in moonroof
S, T or A
The fourteenth is in ham and also chorizo
H
The fifteenth is in gel but not agar
E or L
The sixteenth is in cloudy and also in clear
C or L
The seventeenth is in terrier and not in corgi
T or E
The eighteenth is in hot and also in cold
O
The nineteenth is in elastic but not in band
E, L, S, T, I, or C
The twentieth is in tin and also in foil
I
The twenty-first is in neglect but not in abuse
N, G, L, C, or T
The twenty-second is in Macgyver and also in Angus
A or G
It's pretty easy to find grammatical English words to match the first four paragraphs, but that leaves us with the last paragraph:
DID YOU NOTICE THE [C,L][T,E]O[E,L,S,T,I,C]I[N,G,L,C,T][A,G]
and I cannot get any sense out of it. Perhaps it is:
Anagrammed or otherwise encoded, which makes the puzzle more difficult...
Among the things I've tried:
Adding a missed word break: The only words I can make with this are DID YOU NOTICE THE CEO SING (which sounds like a weird question).
Reversing the positions of the two words in each of these 7 lines (makes [C,L][C,O,G]O[B,N,D]I[A,B,U,S][A,G] for the last word which gives no dictionary words).
Making each of the comparisons wrong (in becomes not in and vice versa): makes [O,U,D,Y][R,I][H,T]A[T,N]E[M,C,Y,V,E,R] for the last word which gives no dictionary words.
Making one of the comparisons wrong (in becomes not in and vice versa): also makes no dictionary words
I'm guessing that:
The thing I need to notice is a seven letter word describing what has been done to the final paragraph
Small Update
@Postmortes has noticed that:
The last letters of each line spell out YOU ARE CLEVER FOR RIDDLES which makes some sense but is ungrammatical... I can't see where to go from here yet.
Also, if we look at which lines:
Have 'in' before the last word and which do not, we get the binary sequence $0111101110111001111111$; I don't know if that is relevant.
I've also tried:
Looking at what connects the comparison words in each line to see if that gives us a meaningful string of words, but can't get anything meaningful out of that - in fact for some of the words it's hard to find a class to which they both belong, as they are more loosely semantically related (e.g. navigator & traveller, optician & optic, email & listserv). Some (like entrance & amaze) have an even more tenuous link (unless amaze is a play on 'a maze').