Oblivion has been the longest time I've ever spent in my gaming career (108 hours before DLC. 130 hours post-DLC). I was actually hesitating to play Skyrim due to the fear of this game sucking my life away. The good news it didn't suck up as much time, only about 70 hours pre-Dawnguard. It was every inch as amazing and epic, but way more manageable.
It does take some time to get used to the Elder Scrolls leveling up system where missions don't really give you experience directly. Your level is tied to your skills. And there are a lot of skills to improve, so you can develop your character however you wish. It allows considerable personalization (unless you're insane enough to try and maximize all skills --- or exploit a glitch to level up nicely) and you can "win" the game in a wide variety of ways.
The storyline is pretty good. But the biggest thing the game pulls off is a truly living world. Dragons will attack you at random intervals (OK, some are scripted, but many are not). There are hundreds of different mines, caves, towns, assorted cottages, etc to discover. The caves avoid the "samey" feeling Oblivion was guilty of which helps since you will be scouring through a ton of them during your playthrough.
You are tasked with helping the Stormcloaks overthrow the Empire...or vice versa. You can join the usual Assassin, Thieves, Magician, or Fighters guilds. You can become a werewolf. You can engage in a mission that has a very "Hangover" vibe to it. You can earn a horse that, on original inspection, seems to be immortal (later experience shows, tragically, that it is able to be killed). There is a lot of stuff to do.
You also definitely feel more powerful as the game goes on as well. Spiders initially are difficult to handle but quickly become cannon fodder. Skeletons worry you, but that doesn't last long either. This is not to say the game gets less difficult as time goes on because the dragons are a pain, giants can be a challenge, and the next to final mission is really, really challenging (the final boos battle, sadly, is really easy IMO).
The game doesn't really have a flaw. I can say that I cannot imagine any improvements that the game can have. The Magicians and the Fighters quests aren't as compelling as the Assassin's Guild and the Thieves missions can grind on for a while. But these aren't really major issues at all and the rewards for completion make up for the occasional boredom of the pursuit.
This is a game that warrants playing. The diamond standard of the RPG world. Possibly the high point of the entire genre and one of the most amazing titles this generation.