We've lost attributes, brithsigns, character classes, meaningful differences between races. We have less player choices, and less variety or diversity in the characters we interact with and the organizations we can be apart of. So from that standpoint a lot of people criticize Skyrim, and have been disappointed or unhappy with it.
You say that as though it is a fact, when in actuality, it is not.
I, for one, believe we have
more choice than we did before.
Attributes-I don't see this as a loss at all. Everything that Attributes covered is still in the game, either through the Health / Magicka / Stamina bars (which is really all that most attributes in past games covered anyways), and what's not covered in the 3 Attributes we do have is covered in perks (I.E.: staggering, damage, carry capacity, etc.) The only thing that's not in Skyrim from past games is Speed, and that is actually done in a different way with sprinting, which while not the
same is really just a different way of accomplishing the same thing.
Which is ultimately the difference between how Skyrim handles it vs. Morrowind or Oblivion. The results are still there, they haven't been removed, they are just handled differently.
You don't like the way it is handled now - which is a totally valid opinion - but it is a subjective one. Claiming that it is not there is an inaccurate statement of fact.
Birthsigns-Again, the effects of the Birthsigns are still in the game, they are just now handled differently in the form of Standing Stones. Again, you don't like the new way of doing it compared to the old, but it is a subjective preference, not a factual lack of content or "RPG mechanics"
Classes-Actually, classes are just the same as before, the only thing missing is the label.
In Morrowind or Oblivion, you pre select a handful of skills that "define" your class, and as such you level up, but there is nothing that limits you to those chosen skills. You are still able to branch out and become any kind of combination of skills you want, including becoming a full fledged master of all. You level up by doing, and there is nothing in game that stops you from branching outside of your "class" except yourself. "Class" was just an arbitrary title that had no impact on the game whatsoever.
Fast forward to Skyrim, where skills level you up by doing, just like past games, and you can focus on any combination of skills you choose, just like past games. The difference is that where in past games, you "defined" yourself pre-game by tagging some skills and giving them a label, in Skyrim you instead define your class as you play by investing perk points in the skills and perks that would define your character.
As such, Skyrim actually has
more of a "class" system, by restricting you more, as you cannot fully perk out every skill, so unlike Morrowind or Oblivion, you actually have to make choices about what skills and abilities will define your character.
Meaningful Difference Between Races-There really isn't any less difference in races than before. If I want to be a magic class, I will likely still play a Breton or High Elf, due to their magical bonuses and extra spells to start off with, if I want to be a warrior class I will still likely play an Orc or a Redguard with their bonuses to combat skills, and Khajiit and Wood Elves still make the best stealth characters because of their bonuses. Every race being able to excell at every style is a trait that is shared in both Morrowind and Oblivion, there is no more "meaningful" difference between the races in past games than there is in Skyrim.
Less Player Choices-I would argue that we have
more choice. When developing your character, you actually have to make decisions on what skills and abilities define your character, because you can't fully perk everything, where as in Morrowind and Oblivion, there was no significant choice save for what skills you wanted to define your arbitrary class tag. But you could still fully branch out and do anything and everything in terms of skills, which meant there was no real choice. Perks in Skyrim allow you to specialize your character in ways never before seen in an Elder Scrolls game. As far as quests go, Morrowind and Oblivion were particularly linear as well, but Skyrim actually offers aspects of choice in its quests, whether it's with Paarthurnax and the Blades, the negotiations between the Stormcloaks and Imperial Legion, how to deal with Saadia, how to figure out the truth behind "A Night To Remember", or any other number of questlines with choice.
Less Diversity-I would argue this as well. In Morrowind, a very large percentage of NPC's you encountered were Dunmer natives of Morrowind that didn't like outsiders. It is no different than the Nords in Skyrim. While Morrowind NPC's had repetitive recycled text dialogue, each voice acted NPC in Skyrim has some kind of unique story or tale to tell. I am much more invested in the NPC's in Skyrim than I ever was in Morrowind. As far as guilds, I feel there is plenty of diversity, where the College of Winterhold is completely different from the Companions, whereas in Morrowind, they were pretty much the same thing outside of the fact that one group did combat and the other group did magic.