Are you sure about that? I'm going to be playing a master thief in heavy armor, a longsword, and a shield.
I notice you guys arguing about a master thief using heavy armor would be bad. Notice how he never mentioned anything about his thievery techniques. There are many forms of thievery, he could take the direct confrontation approach like a bandit who robs people threatening them with his blade, he doesn't necessarily have to sneak around, in which case his heavy armor, long sword, and shield would be very useful. Just some technicalities to be pointed out.
As for the skills I like what NightShadow had suggested for the combat skill, Martial arts or named similarly combining hand to hand and acrobats. I'm not sure about unarmored, it made sense both ways having it (Morrowind) and not having it (Oblivion). Increasing it would mean you'd have a higher armor rating than if it were lower because your body was tougher, but at the same time it felt weird that you had an armor rating without the use of actual armor and that's what having your health was for, increasing that would essentially increase your durability. I'm still not too sure, as for acrobatics and athletics did they mention anything related to how those work? I know they supposedly removed them as individuals, so they may have combined them into something new that covers jumping, dodging, running, sprinting, swimming, basically all athletic feats.
As for the armors they could have combined light and heavy armor, and maybe even threw in medium armor if they confirmed 3 branches per skill, it would make sense bringing it back, sort of. If there was no set number of main branches per skill tree then of course that could easily void the whole argument of bringing back medium armor.
As for the Stealth skills, I think they'll have Sneak, Marksman/Archery, Speechcraft (combination of Speechcraft and Mercantile), Alchemy, Security or something of that sort that affects lockpicking, disarming traps, maybe even creating/setting your own traps(?) though they didn't go into detail much about the stealth skills. They've mentioned the whole 1-handed/2-handed weapons skills with Combat and continued mentioning it in interviews, etc, also the removal(?) of athletics, and the addition of Smithing. They've mentioned that the Magic skills were going to be pretty much the same, minus Mysticism which was going to be broken down and dispersed amongst the other Magic skills, Alchemy being moved to Stealth, and Enchanting being added to it. As for Stealth the only thing I heard them talk about aside from moving Alchemy to it, was that Archery was going to be more powerful and require more time to fire, but was about it.