I've played Elder Scrolls games since Daggerfall. Not going to talk about the other games since this isn't about them. But, I will say I've enjoyed the whole series.
Gameplay system:
Skyrim is great. Brilliant even. Felt like the time I spent on the charater was really worth it. Skyrim has a nice combat system and this is coming from someone who even liked Morrowind and Daggerfall's combat systems well enough.
Main quest:
Started out with a bang. Kill a dragon, take it's power while NPCs watch on in awe and talk about this dragonborn stuff. Really good start. After that, it goes downhill very sharply. I'm going to get into some detail here, so if you're reading this and don't care for a small wall of text then please just pass i up.
Alduin:
"The dragon at the end of time" was a great concept. But, the story behind it was just handled so poorly. So, followers of Akatosh (or Shor as the Nords call him) can do all this great stuff to have a nice place in the afterlife. But, it doesn't matter because a monster can still come to the afterlife and gobble them up, anyway. While Alduin is flying around doing his thing, I'm looking around for Shor, who surely is out trying to protect the souls of his devout followers. No? He's over there sitting in his shiny palace? Why are we following this god again? I'm sorry, but that just feels really weak.
Delphine (aka Super Invincible PMS Woman... forget Goku)
I'm petty sure I simply need only to mention the name. But, here goes. I get the idea that Delphine has an authoritarian personality. Not very blades like, but hey. The only really bad part is how she drops the bomb about Paarthunax then hits you with a "your either with us or against us" ultimatum. WHAT. After all that stuff you did for her, she thinks it's a great idea to hit you up with an ultimatum?
Worst part is you can't do anything about it. No dialogue options to reason with her or tell her off. At first, I thought Delphine was meant to be extremely stupid, but the lack of dialogue options to deal with it prove it's just poor story and script writing on Bethesda's part.
At least the main quest is extremely short. I really shouldn't have to say that. What's wrong with you guys, Bethesda? Surely even you guys could see how poorly implemented some of this stuff was. After all the greatness of the past games, this gives me the sincere feeling that Bethesda is sick and tired of continuing this great series. I hope I'm wrong.
If it means anything, here's my thoughts on the past Elder Scrolls games...
Daggerfall: Great Main Quest. Evil dungeons. Bad level scaling system. After level 20, only enemies you seem to meet are almost just daedra. Lack of variety gets boring.
Morrowind: Best Main Quest. Most well-done game world. Almost everything is important in some way. Game that got the best personal touch. Even the simple bandits are hand placed and have their own individual names.
Oblivion: Phantom Menace of the Elder Scrolls series. Some parts are good. Some parts are.. eh. Oblivion gate, anyone?
Skyrim: Best gameplay system. Really good game world. Bad Main Quest (it was like they didn't even care). I think Skyrim's dungeons are good, despite what I've read elsewhere. Just a touch less linear would be nice, though, just a touch. If they made another game with Skyrim's overall level of quality I'd buy it. Wouldn't pre-order it, but I don't do that for anyone.