I like 3E in that you could learn to be anything, but they made a serious mistake in trying to push "every level is equal" - picking up 1 level in wizard is not of the same value as getting your 7th level of fighter, and the reverse is even truer (though the latter is mostly a fault in the inbalance between spellcasters and non-spellcasters in 3E).
I'd like to see something more along the lines of 2E's dual-classing rules, but that you can switch between class advancement. If your a 6th level fighter and you want to pick up your first level of wizard, it should only take about 1,000 XP - not the XP to be 7th level (as a fighter). However, this has an issue as well - you could abuse it by taking lots of 1 or 2 level class dips for the cost of one higher level in any given class.