I've watched through the series a few times now. It's one of those you have to warm to. When it was first aired I thought that it was terrible and should not have been a Trek thing at all. But after a few views via DVD and its re-run I've gotten to like it. But ... as I have mentioned in previous posts, a real long time ago as I've not posted in ages, the problem ii my opinion was that there was too much temporal stuff. Kirk in TOS had only discovered at that point about the possibility of time travel.
We also met new species which were not seen in later various series, and the inovations of such things such as holodecks were barely mentioned. Did they actually exist in the time of TOS? I can't remember. Then we come to the choice of Captain, should Scott Bakula have been choosen, he's a great actor but not sure he was Star Trek captaincy quality.
These are just a few things, apart from too much time travel most of the stories and action was good. Pity it failed as it was great idea.