Maybe the Modern day Klingon, I loved Michael Dorn's Worf but some times the other Klingon just seemed too cartoonish. How could a feudal, infighting society so infatuated with Knives, Blood, Tribal rituals and Martyrdom ever become an inter stellar species ? 
Perhaps humans can be the most disappointing, maybe Roddenberry's vision was too Utopian, much too pleasant and very rare is it we've seen their urban, dirty, agricultural sides out on the edges of civilization like the Joss Whendon tv series Firefly portrayed humanity. DS9 gave humans a grittier feel but we were still in the Federation's universe. Humanity IMO needed to be portrayed a little more human.
Then we have aliens that appear in all Trek series like the Boslics - WTF are these people about ? Do I care ? Add the Nausicaans etc to this list of Meh! Tribbles seem to have some kind of internet following and appear throughout the series, don't even get me started on these guys.
IMHO Jem'Hadar are worse than the Klingon ever could be, they are a bunch of micro-brains on steroids. Cardassians are IMO a bunch of nazis with absolutely no charm or redeeming qualities.
Ocampa, the Kes species, must be one of the species the "writers" screwed up the most. One of the shortest lifespans, only one child its amazing the species didn't go extinct on day one. It's difficult to be disappointed in a people that never took flight in the first place. The Ocampa come and go with Voyager, their badly written characters never get to plague the ST universe ever again.
The "Q ", great in TNG, but they get deconstructed with DS9 and in Voyager a complete disappointment.
Then we have Species8472 yes they defeated the Borg in a STVoyager battle but ever since this stunt they have been all bark and no bite.
Neo-Romulans ? For me the word Romulan means Tomalak played by Andreas Katsulas from Babylon-5 or Senator Kimara Cretak (played by Adrienne Barbeau & Megan Cole ) and other clever Romulans you see in TOS, not the idiots you see in Star Trek: Nemesis

Perhaps humans can be the most disappointing, maybe Roddenberry's vision was too Utopian, much too pleasant and very rare is it we've seen their urban, dirty, agricultural sides out on the edges of civilization like the Joss Whendon tv series Firefly portrayed humanity. DS9 gave humans a grittier feel but we were still in the Federation's universe. Humanity IMO needed to be portrayed a little more human.

Then we have aliens that appear in all Trek series like the Boslics - WTF are these people about ? Do I care ? Add the Nausicaans etc to this list of Meh! Tribbles seem to have some kind of internet following and appear throughout the series, don't even get me started on these guys.
IMHO Jem'Hadar are worse than the Klingon ever could be, they are a bunch of micro-brains on steroids. Cardassians are IMO a bunch of nazis with absolutely no charm or redeeming qualities.
Ocampa, the Kes species, must be one of the species the "writers" screwed up the most. One of the shortest lifespans, only one child its amazing the species didn't go extinct on day one. It's difficult to be disappointed in a people that never took flight in the first place. The Ocampa come and go with Voyager, their badly written characters never get to plague the ST universe ever again.
The "Q ", great in TNG, but they get deconstructed with DS9 and in Voyager a complete disappointment.
Then we have Species8472 yes they defeated the Borg in a STVoyager battle but ever since this stunt they have been all bark and no bite.

Neo-Romulans ? For me the word Romulan means Tomalak played by Andreas Katsulas from Babylon-5 or Senator Kimara Cretak (played by Adrienne Barbeau & Megan Cole ) and other clever Romulans you see in TOS, not the idiots you see in Star Trek: Nemesis
