Unfortunately, this won't bring back the old-tyme feel of multi-classing. What it will do is make sure that all your adventuring parties are full of single classed (or multiclassed with prestige classes) human characters. Some players would try to buck the trend (by playing multiclassed demi-humans) at first but starting around 11th level the single classed characters will be 1 or more levels (often 2 levels) ahead of their multi-classed companions and it will only get worse from there. Even if players are satisfied with that, the challenges necessary for the single classed adventurers will kill off a lot of the multi-classed characters. The level loss from raise dead/resurrection will then further the imbalance until everyone realizes that multi-classed characters can't compete with their single classed human counterparts.
The reasons for this are two-fold:
1. Multi-classing isn't as powerful in 3e as it was in 2e
2. A 20% difference xp means much more in 3e than it did in 2e
1. In 2e, a multiclassed fighter/mage (what I usually played) would be about one level behind the single classed fighter in fighting ability and two levels behind the single classed mage in spellcasting ability. If an elf (an who wasn't), he could also wear armor (elven chain mail) and cast spells without penalty. In 3e, however the multiclassed fighter/wizard has to make a lot of choices. He can stay one or two levels behind the real wizard in spellcasting ability (by only taking one or two fighter levels), but then his fighting ability will only be marginally better than the single classed wizards' and he'll get cut to pieces on the front line. Alternately, he can keep the levels even and have the fighting ability of a cleric but weak spellcasting ability (he'll be getting Bull's Strength when the single class wizard gets fireball and won't get fireball until the single classed wizard has 10d6 fireballs and cone of Cold). The result of that is usually a weak character who's neither a good fighter nor a good wizard. As a final alternative, he can only take a couple levels of wizard (for shield, true strike, mirror image, etc) and the rest as fighter. In this case, he'll be a defensively oriented fighter who has to spend a few rounds at the beginning of every combat buffing himself. And all these cases suffer spell failure when casting in armor.
Other classes work bettter together (fighter/cleric, fighter/barbarian, fighter/rogue), etc. but none are clearly superior to a single classed character of equal level (fighter/barbarian, fighter/ranger, fighter/ranger/barbarian and ranger/rogue are the possible exceptions). While a fighter/barbarian is a great combination, I wouldn't want to take a fighter 4/Barbarian 5 up against a fighter 11 or barbarian 11.
On the whole, multiclassing in 3e isn't strong enough to support such an experience penalty.
2. This really goes hand in hand with #1. 2e used an exponential experience system that require millions of experience points to reach 20th level. 3e requires a scant 190k. Consequently in 2e, even a 50% xp penalty meant that you were only one level behind your companions and on a few adventures every level, you were actually the same level. In 3e, even a 20% experience difference is much more significant.
Of course if your players are powergamers, they'll just say "20% xp bonus, sign me up" and you'll have nothing but single classed humans from the get-go. (With the bonus feat and bonus skill points, humans are a very advantageous choice anyway in 3e and single classed characters are also quite powerful). The results will be quite different from 2e because IIRC, 2e parties had to be at least 50% elf with a minimum of one bladesinger (2 or more preferred
. In all seriousness though, I don't think this will do what you want it to.
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