A word of warning, don't go overboard on the mods when you first start.
I tried it around a year ago, and decided that in order to maximise my experience I should make sure I get all the best mods installed. So I downloaded over a gig of mods, around 20 in total. I then found that I wasn't really sure where to start, in fact I probably spent more time hunting down mods than I did actually playing the game.
I think the problem was I got roped in by all those comments saying how mod x/y/z was 'ESSENTIAL' or makes the game 'INFINITELY BETTER'. Maybe the best approach would have been to just start up the game in vanilla mode, play for a while getting used to how the game works (bound to make mistakes and want to re-roll anyway I'd imagine), and then hunt down mods to specifically fix any gripes I found before restarting.
Beat me to it. In general you should play the game and decide what you don't like. Then install a mod which fixes it. Avoid the big gameplay-altering mods like MMM or OOO until you are an experienced player. There are really only two mods which could be described as vital:
a) The BT mod, which increases the amount of inventory which is visible when you open it.
b) Immersive Interface, which shrinks the massive HUD.
Everything else is personal taste, no matter what anyone else might say. "Vital" means "I like it anyway". If you feel even slightly brave, make your own mods: pretty much everything in the game can be changed. I've altered the amount of magicka per intelligence, the load-carrying capacity, the speed of arrows and spells, added houses and dungeons etc. And that's without a single line of code.
As to the reasons why people don't like Ob?
First of all there are us experienced MW players, who miss the complexity. Oblivion is a very good game, but MW is a great game.
Many people misunderstand what Ob is. It's NOT a hack'n'slash like DMoMM: there are no combo-moves etc. Fighting is pretty basic, but is designed so that it is your character's skills which win, not your l33t keyboard prowess. As an old man with poor reflexes, I like this, but many youngsters don't.
It's not a true RPG: if you want a character that takes two hours to design then play something else. Characterisation is more for personal RP than for hardcore RPGers.
The game is an adventure game with some RPG elements, and involves a lot of travel. Its greatest plus is the completely and utterly open-ended gameplay. Don't like the Main Quest? Then don't play it - very little will be different.
Play it. Even if you don't like it in the end, you should do it.
Edit: nearly forgot - here are some screenies.
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