There are a couple of things going on with the claims of "dumbed down", and when people oversimplify (which happens often) they're not making a good case for themselves.
First, game marketing has glommed onto the phrase "streamlined". Ideally, streamlined means that extraneous fluff is being removed, but frequently it means "we yanked features but want to make it sound like a good thing". Sometimes it IS a good thing. Sometimes it's not. Regardless, some people get annoyed by marketing hype, and that creates a confrontational atmosphere from the start.
Some people want TES to be a fantasy FPS. For these people, any amount of feature cutting is a good thing, as long as it doesn't affect graphics or animation. But even if you're in the FPS camp, you're not being rational if you think removing features is (usually) not simplifying the game, i.e. dumbing down.
But to make matters more complicated, there are many features in every BGS game that are poorly implemented. TES history is that a badly implemented feature will get removed rather than fixed. Often the idea is good, but the devs screwed it up, probably because it was low priority. Mods will come and fix these features, showing what could be done if the devs put time into it, but instead we see the feature removed from the next game. This behavior creates a lot of contention on the forum.
There are various aspects to gameplay: strategy, planning, role playing, immersion, etc. Some people want challenge from the game, some people want to make choices and think, others want to feel like gods from the start of the game and/or "just play the game". Each release from BGS narrows the range of game elements (i.e. less strategy, less planning needed), and each new game is a little closer to an FPS.
Many of the feature cuts or changes are debatable (or just a matter of opinion), but some aren't. The GPS and quest arrows, for example, are clearly hand-holding for a casual crowd. These aren't about "just playing the game", they're "just skipping elements that require you to think".
And directions in your journal that tell you "take this path, make a left at the rock, and then take that 5 steps and turn right, and there's your quest dungeon" with a big yellow marker on your map is requiring you to think???
And I'm okay with features being removed, when they are replaced with more fleshed out, more in depth and more complex features instead, which is exactly what happened with Skyrim.
Weapon degradation was replaced with a fully fleshed out crafting system.
Spellmaking was replaced with a more in depth casting system and gameplay mechanics (which I admit is not fully fleshed out, but it's better than what we had)
We lost Athletics and Acrobatics, but saw the inclusion of Enchanting and Pickpocketing which add more roleplay possibilities.
We lost questline length, but got much more involved and fleshed out individual quests.
We lost Attributes but instead got a fully fleshed out Perks system that allows for way more flexibility, specialization, and customization for characters.
We gained Dual Wielding which, whether you agree with how it was implemented or not (I happen to love it), it is a CHOICE that we never had before. More builds, more options.
We gained a much more detailed and complex world.
We gained actual puzzles in dungeons to overcome. They might not be overly complex, but they require you to think more than Morrowind dungeons ever did.
There are actually choices in the main quest. They may not be -huge- choices, but it is more choice than we ever had in Morrowind.
The fact is, the claims of "dumbed down" are 100% false - this game actually makes you "think" more than Morrowind ever did. So many times in Skyrim I've gotten lost going from point A to point B, or I've gotten stuck on how to solve a problem in a quest or a dungeon, and spent plenty of time trying to actually figure it out. Something I never had to do in Morrowind. I never had to "think" in Morrowind.
It's a fact, simply put, that Skyrim offers more than Morrowind ever did. It is more complex, and offers you more choice than Morrowind ever did. So you don't have a character stat page, can't create a spell, and NPC's don't respond to your deeds as much as you like. That doesn't equate to "dumbing down", when the game improved and became more complex in every other aspect of the game.