the cool thing about Zer in X1-2(possibly 3 also,) was that he was very awesome in terms of abilities. he could do what X couldnt. he had an autodash. which is probably the biggest thing, had a blue mid charged shot which could destroy ride armors, was red, was like a sort of big brother figure for X, and was not playable, which made you want to play as him. I cant explain it.. its actually like, he was better when he was broken. once he became "standard" he began to lose his touch. i also think that he should have stayed a Buster type. not just because he had a default double shot, but because it underlined the fact that he was Wily's answer to X. Light made a blue sleek kind hearted robot with a buster, Wily made a red, angular and cold hearted robot with a buster. And he had long blond hair. whereas X had no visible hair, which was presumably if he had any, similar to Rock's hair.
the they went and made his saber a main weapon and just dropped the buster like he never had it. teasing us in X5 and 6 with a mostly useless buster which could only be used while stationary on the ground, and had a pitifull range and size. although X6's was quite powerful.
then theres the appeal of his X1 form because we never got to play as it. I still think MHX should have had a Zero mode instead of a Vile mode. because his X1 form looked just so awesome. it was simple, yet very cool.
another thing that kind of made Zero lame IMO, was that they deled too deep into his story. he was mysterious in X4, and Wily was mentioned in his nightmares. X5 kind of introduced Wilys presence to everyone else, and then X6 tried to put Wily right in Zero's face as Isoc. X6 kind of ruined Zero's backstory a bit for me. then after that, Wily and Zero's fears of him and his destiny were completely forgotten, and his character became just way too cold and emotionless. up to X5 and 6, he still showed some emotions. then in X7 made him totally emotionless. X8 following suite. the thing is, if they are going to make him emotionless and expressionless, then dont give him too much dialogue. like the Z series. he's pretty cold there, but he speaks little outside of mission related jargon. like discussing about neo arcadia and what to do about a missile base. he talks a bit with the bosses and thats that.
in other words, they are able to balance cold and personality well in the Zero series so that it works. not quite so in X7-CM.