I'll second (third?) that Oblivion had both way more shallow gameplay and more bugs/broken features/lazy game design than M&B.
Much the same as Assassin's Creed or GTA, it gives a solid impression of open-world gameplay, but is not actually designed to be played as such. They also tried to do some interesting things with their rpg game mechanics, such as skill practice & decay through use or under-use, but they didn't put any effort at all into balancing or mitigating the negative aspects of those features.
Fallout 3 took much of the same direction and improved on some aspects, removed others, and ended up with much the same crap... cool ideas that ended up hurting the game because they didn't develop them enough.
It seems like with every new product they raise hype by inventing cool features, but then they think those features through only halfway and expect their budget to gloss that fact over... which, sadly, works for the majority of gamers. I know tons of people who think Fallout 3 is the best thing ever. Personally, I can't stand it, because I've played other games that have less polish from budget, but much more thoroughly developed gameplay.
We seem to have very different tolerances for one versus the other. I can tolerate some clunky interface, non-game breaking bugs, or graphics that leave touching up to the fan base. In fact, they sometimes add to the charm of a game for me, much the same as a crack in a singer's voice kept in a studio recording can bring a human element to an otherwise highly engineered product. What I cannot stand, however, is being presented with progressive features such as a skill-based rpg system where skills grow or decay based on frequency of use, yet no effort at all is put into addressing the obvious side-effects such as encouraging the player to bunny hop constantly everywhere they go to keep their jumping skill at maximum. Or taking the Fallout franchise into first-person and including the VATS system to stay true to the turn-based rpg roots of the game, but doing absolutely nothing to smooth the break in pacing or balance it so that it doesn't make the game brokenly easy when used properly but the FPS gameplay so clumsy by comparison as to make the game near impossible if you refuse to use it. Or in GTA or Assassin's Creed making gigantic detailed settings with a great superficial impression of life to them, but giving the player nothing to do outside of the core plot besides enacting rampant violence, the evidence of which disappears as soon as you turn a street corner.
Now compare with the economy topic we mentioned before in M&B. What you find to be an overdesigned or superfluous feature, I actually find to be a great method of self-balancing a dynamic system in a way that is consistent with the continuity of the world. For example, I found one city a while back with ridiculously low prices on salt, and another city less than two day's travel away that bought it for a little above standard price. I could make $300-400 profit on every bag... the first time. I ran that route 5 or 6 times, and the profits dropped every time until prices almost totally equalized and it wasn't worth it anymore. I've gone back a couple times, and find it still hasn't gone back to the extreme price difference it was before. I sucked one market dry of salt and saturated another. It will be a while before I can abuse that route again. If this aspect of the game hadn't been so overdeveloped, that discovery would have basically been an "Oh look. I beat the game." moment for me, and I would have lost interest in playing. I can almost guarantee that if M&B were developed by a major corporate studio that's how it would have been, too.
Of course, this is similar to the concept of planned obsolescence in other industries. If a game's gameplay is too well-balanced/interesting/self-sustaining, then the customer is likely to grow attached and sink tons of time into that game, which ultimately means they're probably going to buy less games overall. It's much more sensible business to make a game that can be completed, and leaves little more to do after completion other than go out and buy the sequel.