I'm also doing to suggest BFRPG at higher levels. Swords & Wizardry at higher levels might fit the bill too (ascending AC is optional). Both of those trade a little bit on the "PCs feel mechanically different" front.
I would be wary of suggesting WFRP 2e. It's a great game, but it relies almost as heavily on a battlemat as 3e if you play RAW and, depending on what kind of character creation options you use, PCs can be either really powerful or really fragile to start out with. Plus, it's not really what I would call "traditional" fantasy, although it could be modified to be more Tolkienesque if you wanted to invest the time.