This is how I understand the Bethesda/Bioware thing.
First of all, it should be stressed that both companies have taken long strides away from the Diabloesque grind + loot formula. Those RPGs morphed into MMORPGs and the single-player variety have largely died out, thank goodness. In reality, I don't think either Bethesda or Bioware really care about the leveling and looting part of their games as much as they care about what I've bolded below.
Bioware is a company that cares first and foremost about storytelling, and they are constantly working to perfect that ancient art. Bioware is not interested in making good RPGs, they are interested in making good interactive stories. They use the RPG format as a tool to achieve their goal, but loyalty to the RPG tag is always secondary. That's the only way I can interpret the Mass Effect series' development. Further evidence can be seen in the way they create their levels. Buildings and cities are abstractions with only parts relevant to the story warranting any detail. Almost all levels are linear with a start and an end, just like the chapter of a book.
Bethesda, on the other hand, is primarily concerned with exploration, discovery, and mastery of your environment. Story is always secondary, and is used as a tool to create opportunities for exploration or to flesh out the world. That's why they have had and will probably always have much better side-quests and factions than Bioware. It's also why their main quests usually don't get much love. Every house should have a door, every mountain should be scalable, every cave should have inhabitants or a story behind it. They are all about providing you with a world to fool around in, and the freedom to prioritize what you should do next. The natural downside, of course, is that they often overreach and have to cut costs in terms of NPCs.
As a side note, I don't think Obsidian really understood the Bethesda 'spirit' when they made FNV. Invisible walls are pretty much the polar opposite of what the latter's design philosophy is all about.