To be honest, in my ongoing games I have leveled up characters via "judge fiat" so far -- not a lot of actual XP tracking -- "okay, that's three adventures at level 1 so now you're all level 2!" I consider the XP system to currently be in "philosophical mode," not even playtest mode yet.
In addition to the points noted above, one big "pro" for a unified table is that the rules for multiclassing get a LOT easier. No matter what class you are, you achieve your next level at the same XP increment, so when you level up you just choose what class you take the level in.
That said, I really feel level 5 warriors and wizards (including elves) truly are the most powerful classes at that level. In my games, a level 5 warrior typically attacks first in combat, hits often, and dishes out lots of damage (considering the action die added to damage, various mighty deeds, threat range / propensity for crits, and typically higher results on the crit tables). Wizards and elves are a little more "iffy" due to the spell checks but when the dice are right, man, they can really lay on the damage! A couple good attack spells at a +7 or +8 spell check and wham, you can really turn a fight around fast.
That's where I feel like the different XP per class comes in. Level 5 halfling? Just not as powerful as a level 5 warrior. I don't want to change the halfling class (fundamentally, I mean -- there are some minor things I need to fix)...so how can I justify the halfling player being required to accumulate the same XP as the level 5 wizard??
I haven't figured this one out yet...
I made this suggestion in another thread but moved it here. At a miminum, I'd like to see each entry reduced in cost by a factor of 10 (all thousands becomes hundreds).
Even better, I'm going to use a system like this one to replace tracking XP in large numbers. I got this idea from Monte Cook's World of Darkness which is OGL.
Each adventure (usually 4 hours) the judge awards one XP. Up to one or two additional XP might be awarded but only for a truly extraordinary accomplishment by the characters.
Each XP can buy one of the following character increases in any order:
A. hit points and action die
B. attack and crit
D. class abilities
Cleric: spells known by level
Thief: thieving skills and luck die
Warrior: critical hits and initiative
Wizard: known spells and max spell level
Dwarf: underground skills
Elf: known spells and max spell level
Halfling: stealth and good luck charm
After all four letters are earned (in any order), 1 more XP can be spent to go to the next level and actually increase character level (which also increases caster level if applicable). Erase the letters and start earning XP again at the new level.
If a class is somewhat weaker than another class, that last XP may not need to be earned. For example, a wizard may need 5 XP to get the four advances and a level, but a halfling would only need 4 XP. If multiclassing is an option, the character pays the XP needed for whatever class they are advancing into.
A 0-level character only has to buy the level increase to 1st-level, so only one XP is needed to advance.
The amount of time it takes a PC to advance is up to the GM. Usually a week or so of downtime might be needed. A 0-level farmer becoming a wizard might take a month to polish his understanding of that weird magic he's been having nightmares about his whole life and create a grimoire.