My only experience with Bethesda is Morrowind (which I got long after it was the next big thing) and Fallout 3, so I can't speak to specific complaints about Oblivion particularly.
I actually played Fallout _2_ for the first time relatively recently, and thought that it seemed a lot like Morrowind, albeit with guns, isometric graphics (and really bad, slow combat), and better dialogue. When I found out Beth was doing Fallout 3, I thought, great, all they need to do is get the setting right and write real dialogue, and it should be fine, because obviously they already get the "wander in a vast, well-developed sandboxy world where you can ignore the main quest if you want to." To me, they accomplished these things (they were true to the setting and wrote decent--not the best I've ever seen in the universe, but decent--dialogue).
But I am not looking at Fallout 2--or 3--through the distorted lens of nostalgia, and with all due respect to my fellow gamers who might disagree, I feel that is what is really influencing much (though not all) of the criticism of Fallout 3 in particular. Interestingly, in a few cases, the most ardent critics of Fallout 3 end up admitting they actually haven't played the game (or have only played it minimally), just go by their assumption that Beth could not recreate the Fallout-verse, and they tack on comparisons to Oblivion, which someone else pointed out is popular to bash, for good measure. Others have played it, but are comparing it to an impossible standard created by memories of a game they played a decade ago, and absolutely NOTHING can be seen as good when filtered through that kind of lens.
This is not to say Fallout 3 isn't without flaws, but in a way, all the complaining about how it isn't like the good ol days and comparisons to Oblivion simply obscure valuable discussion of the real problems with the game (specific bugs, etc.). And even if say, Oblivion's main quest was also poorly developed as Fallout 3's is in some places (I don't know if it was because again, I haven't played it), it's better criticism to just try to talk about how Fallout 3 could be improved than continually wailing on about the past.
I have also dabbled on a very small scale in some gamewriting, mostly with editors that have come with other games, and even just writing one single, good conversation tree can take a long time; writing several takes forever. When I look at Fallout 3, even with its flaws, it's hard not to see the number of hours poured into writing and scripting various circumstances; most quests (oddly except perhaps parts of the main quests) have multiple solutions, which is reflective of a lot of both planning and writing time. Graphics were intricately well done--care was taken to make sure the smallest reflection works right, and again, that takes time. So at least for me, I look at Bethesda and while I might see possible room for improvement, I still ultimately see a developer that really has sunk a lot of time and effort into a HUGE game that I find after several weeks of play still completely engrossing. So I can't complain about that.
My only, personal, real complaints about Bethesda lie in the PR and marketing. Their rep, Pete Hines, promised "no invasive DRM" only to include SecuROM software on the disc---yes, it turned out, only the version with a disc check, but given that was specifically the software that was making so many people paranoid about invasive DRM, it was an extremely poor choice on their part to go with that product, because it only resulted in hysteria around the release of the game, when the first SecuROM related bugs were reported. They also made some comments about things like the game's ending which were also clearly exaggerated. I think they also made a poor decision--from a gamer's perspective, not a business perspective, mind--to include Windows Live, since that has also caused game load problems and many are annoyed that they will need to subscribe if they want to purchase DLC (and I am curious to see how that will affect sales. It may not, but it will be interesting to see). Finally, I think the marketing of the game itself was poor, focusing far too much solely upon the combat aspects of Fallout, making it look like an FPS when it is a fairly indepth RPG. I realize just a trailer full of conversations and skill checks wouldn't be very interesting, but there should have been a better balance to show what the game really was about. 6 different demonstrations of VATS isn't that interesting, and is moreover misleading.
So, overall, I'd say the Devs should keep working on improving what they already do extremely well, but they desperately need to get some new PR folks. And fire Pete Hines. And maybe some better QA testers while they're at it, so things like the Live bug wouldn't have gotten through.