Being made twenty years after TOS may have something to do with the improved special effects, costumes and props, though they are all based on TOS. That and being the most expensive show on TV at the time. The writing and characterization come off as incredibly bland, especially after Roddenberry was no longer part of the day-to-day process. Plus, the show was no where near as challenging on the idea front as some of the network offerings of the day (nor TOS for that matter).
That's why I wrote "not just special special effects, props etc." that should be a given and expected, however that the writing improved so much as it did in my eyes, was not a given and wasn't present, again in my eyes, in at least the first season.
And yes it was based on TOS, but everything is based on something. They were inspired by some designs and ideas and improved on them. In some ways they could have utilized more on the earlier ideas such as making incredibly human looking Deanna a human esper (as introduced in TOS' pilot) instead of an alien.
I actually think many things improved (in TNG at least) after Roddenberry was less involved in it. Case in point: Troi's character who became much more competent again and got episodes like "Face of the Enemy" and "Dark Page" Worf got much better after season 1 and continued to improve. Riker stopped posturing and jumping around like an over-emotional monkey after season 1 etc.
The one thing I'm really disappointment in is that we didn't get a homosexual character as Roddenberry apparently planned shortly before his death.
What I'm really thankful for in TNG is that we got away from the family-less swingers of TOS. I know not everybody liked the idea of children and families being present, and it was nearly not as fully realized as it could have been (Riker and Troi should have been married and with child by the end of the series) but I think we met at least one family member of each major character by the end of the series with several of them being recurring characters such as Lore, Lwaxana and Alexander.
To me their family are an important part of a character, it helps me understand them and makes them more interesting in my eyes.
I agree that the characters in TNG could be bland and lifeless at times (but again I would put those times largely into the beginning of the series) but was that not in part caused by Roddenberry trying to disallow interpersonal conflicts to show how humans had evolved?
Yes TNG did not challenge as much as it could have (again, why no homosexual crew member?) but it did have its profound episodes such as "The Outcast", "Half a Life", "The Host", 'Dark Page" "Measure of a Man" "Suddenly Human".