There are 3 factors in this game which make it great, and overcome its weaknesses
The game has some shortcomings, but there are 3 factors which really make it great.
1. The game world is incredible. Immersive. It's really fun to wander around in this environment. You have a great sense of having a whole world to explore and play in. I never got tired of the depictions of the great outdoors, the excellent graphics and the freedom to wander through it all.
2. The story is also incredible. It's not so much the story itself, which is well-told and interesting throughout for the most part, but its the way the story unfolds in this world. You can pursue the main quest--or not pursue it. There's plenty to do in this world either way. It reminds me of that old game Wasteland, which was one of my favorites, because of the ability you have to explore the world and pursue the story in a way that seems open and free. It's the way a story in a game like this SHOULD unfold (but so rarely does). At the same time you don't ever reach a place of feeling lost or like you can't progress to the next step. It's a great balance. The story is involving but not linear, and therefore a real triumph.
3. The mods. The way Bethesda has opened this game up to modding has been a huge success. There are thousands of mods out there, and more all the time. I've never quite seen anything like it before. Most problems with the game are fixed by some mod or other, rendering the game's shortcomings relatively insignificant. And it's like you're buying multiple games in one!
Well, as I mentioned the mods largely overcome the games weaknesses, but the weaknesses are probably worth noting as follows:
1. Strictly as an RPG, this game is a failure. I was content to play, the first time through, as a blade-wielding, heavy-armor wearing warrior. That was fine. But I found out afterwards that, without some good mods to change it, you really can't play this game successfully any other way. If you want to be an archer/thief, for example, forget it. You get to about level 7 and you're getting killed left and right by every stinking rat (literally!).
What causes this failure is the terrible leveling, which penalizes you for building up the very skills you selected when you designed the nature of your character. If you create a mage and spend your time doing magic, you are lost. The same with a thief, an archer, an acrobat, a monk, or whatever. You'll have to go back to your sword and your heavy armor, but by then you probably need to start over.
And all the creatures and characters in the game level up with you. Instead of you getting more powerful, you end up playing a game of keep-up: keep up with the other creatures or you'll get creamed. Thankfully there are a number of mods which address this, and practically all of them are better than the original method.
2. The game suffers from its own longevity. The sounds, especially the voices, are excellent, but begin to become repetitive. There isn't enough variety in them for 100+ hours of play. Yet I can hardly fault it much for being so successful a game that I'm willing to play for over 100 hours. Graphics suffer from the same problem. They are beautiful and technically superb, but after a while everything starts to look alike. I never got tired of the great outdoors in this game, but I did get tired of the interiors and the characters and creatures.
3. The interface suffers from XBox compatiblity syndrome. This interface for the PC should have been way better, but it was obviously designed for the console. Seems like such a waste, especially when a few modders came up with a much better interface in their spare time, putting the game designers to shame. Hope Bethesda does better on their next cross-platform game.
There are other shortcomings, but those are the main ones. Thankfully, there are mods in abundance. I highly recommend this game, but expect to spend quite a lot of time playing it.