It does lead to an odd thing that the "value" of +1 ceases to be +5% and varies depending upon what number you want need to roll. I believe it gives the "most percentile" benefit when you need to roll a 10+ or 11+ on the 3d6.
Also could lead to very tiresome fights between high AC monsters and players trying to hit each other.
If you're rolling D20+4 to hit AC: 20 you hit on 16-20; so 20% chance or an average of one hit in five attacks.
If you're rolling 3d6+4 to hit AC: 20 you hit on 16-18: so 4.62% change, so on average less than one hit in twenty attacks.
Threat Range Maths! Wheee!
The other thing is critical threat ranges (and similar) need adjusting to accommodate the new probabilities.
Rolling a D20 has 20 different possibilities, rolling 3d6 leads to 216 different ways the dice can fall.
This is useful for corresponding probabilities.
http://www.thedarkfortress.co.uk/tech_reports/3_dice_rolls.htm#.VpMFpxWLS70
I believe
3d6
18= 0.46%
17-18 = 1.85%
16-18 = 4.62%
15-18 = 9.25%
14-18 = 16.20%
13-18 = 25.92%
12-18 = 37.50%
11-18 = 50%
Compared to the rather simpler:
20 = 5%
19-20 = 10%
18-20 = 15%
17-20 = 20%
16-20 = 25%
15-20 = 30%
Which actually is a lot better mapping than I first guessed
(although 17-20 is awkward)
So equivalence:
"Natural 20" = "Natural 16-18"
"Natural 19-20" = "Natural 15-18" (Long Swords/Short Swords etc)
"Natural 18-20" = "Natural 14-18" (Rapiers)
"Natural 17-20" = Uh "Natural 14-18 and flip a coin with a natural 13?"
"Natural 16-20" = "Natural 13-18"
"Natural 15-20" = Again somewhat awkward "Natural 13-18 and flip a coin with a natural 12?"
Similarly a "Natural 1" on a D20 has similar probabilities to "Natural 3-5"
Apparently Improved Critical/Keen Weapons helpfully get Critical threats into the awkward numbers "A threat range of 20 becomes 19-20, a threat range of 19-20 becomes 17-20, and a threat range of 18-20 becomes 15-20."