Wikipedia has a section on "Critical reception". Quoting the negative parts:

• Gary Arnold and Judith Martin of The Washington Post felt that the plot was too thin to support the length of the film.

• Time‍'​s Harold Livingston wrote that the film consisted of spaceships that "take an unconscionable amount of time to get anywhere, and nothing of dramatic or human interest happens along the way". Livingston also lamented the lack of "boldly characterized" antagonists and battle scenes that made Star Wars fun; instead, viewers were presented with lots of talk, "much of it in impenetrable spaceflight jargon".

• David Denby said that the slow movement of ships through space was "no longer surprising and elegant" after films such as 2001: A Space Odyssey, and that much of the action consisted of the crew's reacting to things occurring on the viewscreen, which the New York Magazine critic considered to be "like watching someone else watch television".

• Arnold felt that the acting of the main cast (Shatner in particular) was poor; "Shatner portrays Kirk as such a supercilious old twit that one rather wishes he'd been left behind that desk", he wrote. "Shatner has perhaps the least impressive movie physique since Rod Steiger, and his acting style has begun to recall the worst of Richard Burton."

• Many critics felt that the special effects overshadowed other elements of the film. Canby stated that the film "owes more to [Trumbull, Dykstra and Michelson] than it does to the director, the writers or even the producer". Livingston felt that Trumbull and Dykstra's work on the film was not as impressive as on Star Wars and Close Encounters due to the limited amount of production time. Godfrey called the effects "stunning", but conceded that they threatened to overpower the story two-thirds of the way into the film. Kroll, Martin, and Arnold agreed that the effects were not able to carry the film or gloss over its other deficiencies; "I'm not sure that Trumbull & Co. have succeeded in pulling the philosophic chestnuts of Roddenberry and his co-writers out of the fire," Arnold wrote.

• James Berardinelli, reviewing the film in 1996, felt that the pace dragged and the plot bore too close a resemblance to the original series episode "The Changeling".

• The slow pacing, extended reaction shots, and the film's lack of action scenes led fans and critics to give the film a variety of nicknames, including The Motionless Picture, The Slow Motion Picture, The Motion Sickness, and Where Nomad [the probe in "The Changeling"] Has Gone Before.

All of this, naturally, is well referenced; there are plenty of citations you can go and look at if you want more detail.

Wikipedia has a section on "Critical reception". Quoting the negative parts:

• Gary Arnold and Judith Martin of The Washington Post felt that the plot was too thin to support the length of the film.

• Time‍'​s Harold Livingston wrote that the film consisted of spaceships that "take an unconscionable amount of time to get anywhere, and nothing of dramatic or human interest happens along the way". Livingston also lamented the lack of "boldly characterized" antagonists and battle scenes that made Star Wars fun; instead, viewers were presented with lots of talk, "much of it in impenetrable spaceflight jargon".

• David Denby said that the slow movement of ships through space was "no longer surprising and elegant" after films such as 2001: A Space Odyssey, and that much of the action consisted of the crew's reacting to things occurring on the viewscreen, which the New York Magazine critic considered to be "like watching someone else watch television".

• Arnold felt that the acting of the main cast (Shatner in particular) was poor; "Shatner portrays Kirk as such a supercilious old twit that one rather wishes he'd been left behind that desk", he wrote. "Shatner has perhaps the least impressive movie physique since Rod Steiger, and his acting style has begun to recall the worst of Richard Burton."

• Many critics felt that the special effects overshadowed other elements of the film. Canby stated that the film "owes more to [Trumbull, Dykstra and Michelson] than it does to the director, the writers or even the producer". Livingston felt that Trumbull and Dykstra's work on the film was not as impressive as on Star Wars and Close Encounters due to the limited amount of production time. Godfrey called the effects "stunning", but conceded that they threatened to overpower the story two-thirds of the way into the film. Kroll, Martin, and Arnold agreed that the effects were not able to carry the film or gloss over its other deficiencies; "I'm not sure that Trumbull & Co. have succeeded in pulling the philosophic chestnuts of Roddenberry and his co-writers out of the fire," Arnold wrote.

• James Berardinelli, reviewing the film in 1996, felt that the pace dragged and the plot bore too close a resemblance to the original series episode "The Changeling".

• The slow pacing, extended reaction shots, and the film's lack of action scenes led fans and critics to give the film a variety of nicknames, including The Motionless Picture, The Slow Motion Picture, The Motion Sickness, and Where Nomad [the probe in "The Changeling"] Has Gone Before.

All of this, naturally, is well referenced; there are plenty of citations you can go and look at if you want more detail.

In short, the main criticism is that everything was too slow.