Higher power level and more options. My Baldur's Gate II campaign is up to 11th level (after 55 sessions and close to 2 years) and they are starting to have the power level to successfully pull off fun crazy stunts and really start to see payoff with "we are substantially more powerful than we used to be."woah. i don't think i've heard of anyone like this before. why do you prefer 13+?
also, i'd say mid level is levels 7 to 12, personally (that's the 4th to 6th level spell range).
Don't forget in AD&D it was also Dungeon Level and Monster Level and I think a few other "Levels"... it was a bit annoying then.I am just realizing that having both spell and character progression use the term "Level" is actually very confusing at times.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.