Ok, there seem to be a few threads in there that need to be disentangled, but I'll do my best. I may just end up saying stuff you already know - if so, I apologise, but I'm not entirely sure what information you are looking for, so I'll just throw a whole bunch out there, and hopefully somewhere along the line you'll get what you're looking for.
1. Skills are no longer divided into major/minor. Now increasing
any skill will contribute to leveling up. However, to promote specialisation, increasing higher valued skills will contribute more towards leveling up. So, for example, if you increase a skill from 34 to 35, that might get you 15% closer to leveling up, whereas if you increase a skill from 12 to 13, that might only get you 10% closer to leveling up.
2. On attributes. There has been speculation that attributes are out of the game. But this is unconfirmed. However, I seem to recall reading/hearing somewhere that what Bethesda wanted to do with the leveling system is remove that micromanagement you mentioned - where you try to manipulate your skill increases in order to get the greatest bonuses to your attributes when you level up. This created a tension between two things: (i) progressing in the game, and (ii) the "be what you play" ethos Bethesda promotes. The attribute multiplier system (together with level scaling) meant that if you tried to "be what you play", that tended to come at the cost of progressing in the game, where if you played to maximise your ability to progress in the game, you tended to have to play out of character. It seems like Bethesda want to remove that tension. It's not clear whether that means that attributes are out, however. Some people have speculated that attributes are in, but will be more like a GCD type system, where attributes increase passively as you increase the appropriate skills. Others think the job attributes did will be done by other means (some combination of race, skills, perks, health/mana/stamina). Have to wait to see.
3. Level scaling. I didn't play Fallout 3 either, so this is second-hand knowledge. But hopefully it's accurate. It sounds like these are the major differences from Oblivion.
(i) The game will feature higher and lower leveled areas. So for instance, one dungeon might be leveled between 7 and 13, whereas another might be between 18 and 25. The game sets the level of these dungeons as close as possible to your level, when you enter. So, suppose you are level 4. Then the first dungeon will be level 7, and the second dungeon level 18; if you are level 12, then the first dungeon will be level 12, and the second will be 18; if you are level 15, then the first dungeon will be level 13, and the second 18; if you are level 24, then the first dungeon will be level 13, and the second 24; and so on. Also note that once you've entered a dungeon, the game fixes the level of that dungeon for the remainder of the game. But because dungeons will vary in their levels, you can't just run through the game and set all the dungeons to level 1; also, this seems to motivate making your character more powerful, because if your level 6 character gets killed in a level 7 to 13 dungeon, you can just level up a few times, and then come back to complete the dungeon.
(ii) Creatures generally have a fixed level. So you won't find the same enemy in a level 3 to 9 dungeon that you will in a level 25 to 30 dungeon, just with higher stats - or at least, if the creature from the lower leveled dungeon does appear in the higher level dungeon, it'll have pretty much the same stats as it did in the lower leveled dungeon. Higher leveled areas will either have stronger types of creatures, or they'll just have higher numbers of level level creatures, or some combination of this. Similarly, bandits won't somehow get a whole lot of fancy and expensive armour as your level increases.
(iii) Loot will scale in pretty much the same way as creatures. So if you go to a level 7 to 13 dungeon, you'll find loot appropriate to characters of that level, even if your character is level 30. There should still be some hand placed loot around, however.
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I think that about covers it? Hopefully that helps.