While mostly focused on graphics, gameplay is seriously flawed - but customizable.

You'll start thinking that this game is amazing - even on my old PC I thought it was great. But it takes a few hours to run into the downsides - which are many.

It seems mainly based on graphics - look at the screenshots for proof. But if your computer is not up-to-date, forget the screenshots.

Now, the game looks like a mix between a FPS and a RPG. Don't get me wrong, this is a really nice touch. If it's properly carried out. But unfortunately it isn't.

First off, Oblivion completely lacks localised damage. That means, a hit to the head is like a hit to the toe. This both with arrows and with melee strikes. For this same reason, armor pieces will not protect a specific part of the wearer's body. Instead, they will provide a (small) amount of generic protection. No matter where you hit - If you hit someone's head, and he is wearing only a pair of boots, he will oddly get some protection.
Again, hitting enemy armor with weapons and spells won't damage the specific parts of it, but just a randomly picked piece.

To continue, combat is monotonous. For ALL melee weapons the animation is the same - one handed or two handed groups. To the strictly gaming part, it works simply left click, go back, right click, left click, repeat.
No, you can't fight while riding - that could have been a cool addition.
Plus for some reason ranged weapons seem to be REALLY underpowered. You'll take something like 20 hits to take down an unarmored humanoid.

Fight will take a lot, and definitely feel not "lethal". More like a matter of repeated hit\run\cast thing that soon becomes an annoyance.

Also weapon materials are too linear: the more advanced material will be ALWAYS tougher, more damaging AND heavier. Let alone that there are no long weapons.

Now that I described the action flaws, i'll pass to the RPG ones.

The real pain: level scaling.
This quite destroys the RPG-ness. (Almost) All your enemies, all their equipment, all the loot you find, will "evolve" adequately to your level. What's the point of leveling up if it will make no difference at all? Just so that a cooler armor set appears in some random loot? Indeed, it's unnecessary to level up. You can complete the main quest as well as the guild quests as a level two ( not a level one though :-) ). Actually it's easier, since enemies have an extra handicap at the first four levels.

Now comes quest linearity. Same input, same output. That for 99% of quests. Including the main one.
One more annoying issue is given by "Essential" characters. You can't, can't kill them. Just make them unconscious. Couldn't the makers bother to make at least a "Mission failed" screen? Or to break quests?

Thirdly, there are quite a lot of pointless skills.
Light or heavy armor skill, for instance. How is some armor going to protect you three times as much only because you feel more "comfortable" in it?
Or even pointless in terms of usefullness in game. Speechcraft and security for instance. You'll play fine without caring of them.

Also you can't make your own weapons, but that may be asking for a little too much.

At this point you'll wonder why I gave this a 7.5.
It's because of mods. You'll find plenty of them, and you can even make your own fairly easily with a program, TES-CS.
Some of these mods actually fix the most annoying issues in Oblivion, like level scaling, while others add a lot of content to the game. I'd suggest, if you're going to get the game, also get TES-CS and try modding it - it's worth it.