The Elder Scrolls IV: Oblivion is now my favorite PC game, and I would go so far as to consider it the best PC game.
The quests are another major part of Oblivion that make up the experience. There are about 7 or 8 main questlines as well as a main quest that you can play through. You can work for the Thieves guild, Fighter's Guild, Mage's Guild, Dark Brotherhood, the arena, and a few others. Each of these are like several different games rolled into one. I, currently, after putting 37 hours into the game, have only beaten the Thieves guild questline, the arena, and the main quest. The Thieves guild questline is sort of like a stealth game, something along the lines of a medieval version of Splinter Cell. The main quest on the other hand, is a lot like an adventure game like Zelda where you have to obtain a certain amount of items in dungeons to vanquish a boss at the end of the game. The arena was sort of like a Street-Fighter-esque game where you are pitted against a foe in an arena and become more and more respected as you win matches. The gameplay in all of these questlines remains the same, but you have to play through them in different ways. I know that sounds counter-intuitive, but you have to play the game to understand. Excluding these main questilines, almost every other citizen in any town has something for you to do to help them. While these aren't as fun as the main questlines, they certainly make a great way for you to level up your character along the way.
As you play through the game, you meet a lot of characters, some memorable, and some not. Despite the fact that the majority (probably 65%) of the NPCs in the game are just "filler NPCs," that walk around in the town, you actually do develop some attachments to some of the characters that you work with in the questlines. The only major gripe I have with the game is the voice acting. No, it's not really bad voice acting. In fact, I think a lot of it is really good. The voice actors do a great job at portraying their roles. The only bad thing is, there's only about 6-8 voice actors. Given the fact that the game has literally hundreds of NPCs, you'll often find that some characters have the same voices as others. The Gray Fox, the leader of the thieves guild, for example, has the same voice as the Imperial Guards. In addition, one thief in the thieves guild has the same voice as a queen in another province. I feel like they should have hired more voice actors. Luckily, Skyrim supposedly has about 80 or 90 voice actors, so that's probably fixed in the next game.
While I could elaborate a lot more on my experiences in the different questlines, I want to conclude my review here. To sum things up, The Elder Scrolls IV: Oblivion is a game I would recommend to anyone. It's extremely fun, and the game could easily provide at least 200 hours (I'm not kidding) of game time. So, if you've never played Oblivion, you don't know what you are missing out on. You need to get it. I got the Game of the Year Edition Deluxe version on Steam for $6.00 during a Midweek Madness sale. I think that's the best value I've ever gotten for a game in my life. Regardless, you can claim that same package for $25 on Steam, and even that is a great deal. I would buy this game at full price if it was 50 or 60 dollars. I'm looking forward to playing Skyrim in a few months, after I've played Oblivion for all it's worth, and believe me, it's worth a lot more than $6.