I really need to get back to Discovery at some point after watching the first two episodes. I'm curious to see what's been happening since I'll see IGN rate episodes in the 8 - 9 range and AV Club seems to have them around a B level yet oohhboy seems to be scorching every latest episode so who is correct?
As for Voyager, bland is probably a good way to describe the series. It just often seems derivative of what came before although once in awhile it can suddenly muster a spark of genius. One main reason that I would point to is that the Voyager doesn't seem interested in exploring or developing the characters much and is more interested in story concepts or bringing up some kind of moral quandary. That's not necessarily a bad thing but a lot of time they have a lackluster resolution or no resolution at all. At the same time, when they have delved into the characters, the results are often mixed but even though it has produced some weak episodes it has also produced some of the strongest in the series as well. One of TNG's strength's are the main characters on it. You could have an episode with them all sitting around a table in Ten Forward sharing drinks and shooting the breeze for an hour and that would probably make for a really good episode because they've got such good comraderie and rich backgrounds to tell stories from and for a writer to make such a premise work. Try doing that with Voyager's crew and a writer could have a tough time. On the one hand, you could come up with all manner of background stories for them to tell since they're pretty undeveloped yet at the same time they do have some established personalities that you have to reconcile with whatever you come up and work with those personalities bouncing off each other and I think that challenge would drive a writer nuts. They're just kind of one note characters.
Take Janeway, for instance. She's one of the more developed characters by default of being the captain and main character so she's going to factor into a lot of the show. Yet, even though we get an occasional glimpse into background, there's so much left undiscovered. With Picard, we got to see his family and how his brother resents him and his Starfleet career. There was also some past history and flirtation with him Dr. Crusher. With Sisko, he visited his father in New Orleans and had his son with him. We know what happened to his wife and there was the relationship with Kasidy.
With Janeway, she's engaged to a man, Mark, at the start of the show but we don't really get a lot of details about that relationship. We get a bit of it in the first episodes, there's another episode where she "hallucinates" a bit about him and later when they are finally able to get word back to Earth of their survival she learns he has moved on and the relationship is over. Other than that, she's pretty much a spinster. Did she have any other relationships before that? Any other loves that didn't work out? There are a couple times on the journey where she has sort of admired or flirted a bit with someone they've encountered but it has been nothing of major consequence. With the circumstances of Voyager what they are, she's not really willing to date a member of the crew so being able to do much with the character in a romantic relationship is pretty much a dead end which is a shame since that can be a great way to develop a character. It's one of the most common types of a story found in most shows and movies.
Even with her family history, we know her dad is a scientist and that was the big influence that got her into science. There's an episode with an alien that impersonates her father and that back and forth gives us some glimpse into her youth but it's one episode and isn't further explored which is a shame as I do think that is one of the better episodes in the series. What about her mother? I can think of any instance where she has been brought up. You'd think the mother/daughter relationship would or could be something to shade Janeway's character with especially since she sort of has a mother/daughter relationship with Seven of Nine. Heck, she's sort of got a mother-type relationship with other crew members like Harry Kim. Does she herself mirroring her in her actions with the crew? Or are there traits and behavior she is trying to avoid copying or imitating? Now that Mark has moved on, is there anyone else on Earth that she is hoping to see again if they get home? Is there anyone else waiting for her to give her more motivation besides doing it for the crew since it was her order to destroy the Caretaker array? If there is, Voyager don't care. It's got another time plot device it wants to have the crew engage in.
Heck, here's another quick and easy example. In the pilot episode, Tom Paris was kicked out of Starfleet. He tells Harry Kim a quick version of the events that basically go along the lines that something went wrong but it was covered up and he could have escaped punishment. He decided he couldn't go along with that and turned himself in and got booted out. Did the writers have an actual story in mind with more details to help form the background of Paris' character? If so, why not spend some time in episode finally getting into that? Maybe parallel it with current time predicament Voyager and Tom find themselves in. But no, after five seasons of the show, what happened to Tom and getting booted from Starfleet has yet to be further elaborated on.
TNG and DS9 did a lot more with putting characters into relationships and pretty much gave all the characters a family history even letting you meet most of them over the course of those shows. Even though the premise of Voyager makes it difficult to allow for family visits, you'd think the characters would talk about them more. Perhaps create holodeck versions of them or create log/journal entries for them. The most developed family relationship is the Wildman's who are infrequently appearing characters which is pretty sad. Yet, that is why Voyager probably comes off as bland and lower tier in Trek series.