Then thats just another sub-menu I have to navigate through, though it si a good uidea, of course deleting spells is a must.
Not necessarily. They could have all spells visible on the same menu, but have tabs for spells of different levels, with the option to close or open said tabs.
Still, I wouldn't complain about some way to remove unneeded spells. I mean, a warrior can throw away or sell old unneeded weapons so they no longer clutter up his inventory, so why should mages be forced to lve with lists of dozens of spells they don't want to use anymore? Admitably, it's not entirely realistic since spells are learned, you can't just say "I don't want to use this spell anymore!" so you erase it from you're memory entirely, but I don't mind a sacrifice of realism to avoid insanely cluttered spell lists. Then again, the favorite's menu might render this irrelevant as you can just add the spells you use to it and removed the ones you don't.
How would this system handle spells that have several effects? Or custom spells (if they exist anymore)?
At the time of the podcast, at least, custom spells were not in the game, so unless that's changed, we don't need to worry about it. Multiple effect spells could be a problem, though.
I'm still a bit confused on how this spell system is going to work. Is there one generic version of each kind of spell, that just gets stronger as you raise your destruction skill? So each spell in your spellbook is a unique spell?
We don't really know that, as far as I know. We know that players can place a spell in each hand, or carry a weapon in one hand and use a spell in the other, we also know that Bethesda has aimed to increase the variety between different spells with similar effects, mentioning examples of things like a fire spell that acts like a flamethrower or a spell that allows the player to place a rune on the ground which causes elemental damage on enemies that step on it, I think it's also mentioned that the three types of elemental damage have different effects, but I don't recall anything being mentioned about spell progression. And if spell making has been removed, how you progress from weaker spells to stronger spells will be especially important, because in previous games, if none of the default spells are strong enough, you can just make a better spell, as long as you have enough magicka and skill to cast it. In Skyrim, Bethesda will have to ensure that at high levels, you can still find premade spells strong enough for you're needs. How this will be done we'll just have to wait and see.