Heaven forbid his chance is not 100%.
Note: I think that 4E will have opposed rolls or some such on Tumble, so the DC will not be as easy as 3E. Or at least I hope it is no longer a static DC.
I picked Tumble just as a random skill.
Let's pick Perception instead.
Rogue NPCs same level as PCs.
The PC Rogue spots them ~50% of the time because, quite frankly, 4E is now set up that way.
The PC Fighter, Cleric, and Wizard, although their Perception modifiers have been climbing, are not in the ball park of the NPC Rogues.
NPC's Hides: D20 + 10 + x
Rogue PC's Perception: D20 + 10 + x
Other PC's Perception: D20 + x
where x is other modifiers (including levels) and could be +/-0 to +/-3 points for each creature.
Basically, even though all of the PCs have been adventuring for a long time, the Rogue is great at Perception and the rest of the PCs (relative to same level enemy Rogues) suck (assuming they did not take the skill). They have a 40% to 60% less chance of spotting their enemies. Hence, they get ambushed nearly every time and the PC Rogue gets ambushed 50% +- of the time.
I just think that +10 is too great of a delta on a D20. +6 for your best ability is solid but not overwhelming. +10 mathematically is overkill. IMO.
And, it's not just a straight 50%. Let me show you the actual math for one creature having +15 with a skill and his opponent having +5 (assume ties are re-rolls). The opponent's chance to win the opposed roll is:
Code:
rolls %chance to win
+15 +5
1 45% + 5%*23.6% (give or take, I won't bother with multiple ties)
2 40% + 5%*23.6%
3 35% + 5%*23.6%
4 30% + 5%*23.6%
5 25% + 5%*23.6%
6 20% + 5%*23.6%
7 15% + 5%*23.6%
8 10% + 5%*23.6%
9 5% + 5%*23.6%
10 0% + 5%*23.6%
11 0%
12 to 20 0%
The guy with +15 really only has to roll 11 or higher to guarantee a win, but he wins the majority of the time anyway.
He wins ~76.4% of the time and his opponent wins ~23.6% of the time. Now, 3 to 1 might not sound that bad, but that does not include extra bonuses. Chances are that most creatures will have many of their best skills and abilities with higher ability score modifiers. Just changing this from a 15 to 5 example to a 16 to 5 example (a mere +1 on an ability score) changes it to a ~81.1% to ~18.9% or about 9 to 2.
The problem is that +10 to the delta is too close to the edge of most of the D20 die itself. It's too big a piece of the pie mathematically.
Look at the 3E saving throws. There is only a base +6 delta there (not +10) at 20th level and people were complaining about how "the bad saves" were so bad that PCs nearly always fail them at high level.
For the same reason that +6 +ability scores +magic is too big of a delta in 3E for saves, +10 +ability scores +magic (hopefully less magic) in 4E will be too big of a delta for opposed skills.
I hope WotC realizes this and takes a real close look at the math.