yesso in DDO we roll a twenty sided die (d20) when we attack. every weapon base will generate a critical hit¹ on specific values of that d20 - this is also known as "threat". every weapon base also has its own base damage dice and critical multiplier. let's look at the great axe and falchion:
great axe has base damage dice of
1d12, or a twelve sided die rolled once, therefore a +4 great axe with no other damage modifiers will do between 1+4=
5 and 12+4=
16 damage. its threat is only on
20, which as one out of twenty = 5% is fairly described as "low". its multiplier is
3, which is larger than the minimum value of 2 so i suppose it could be called "large".
falchion has base damage dice of
2d4, or a four sided dice rolled twice, therefore a +4 falchion with no other damage modifiers will do between 1+1+4=
6 and 4+4+4=
12 damage. its threat is on
20, 19, and 18, which i wouldn't say is a "good" chance in a vacuum but it is the highest chance of any weapon base. its multiplier is
2 which is the minimum, so critical hit fairly has no adjective.
the first thing is there's no way to mathematically compare low * large against good * regular. so forget the words entirely, your best bet when starting out is really to just fire up a spreadsheet and write out what happens for each roll of the d20 from 1 to 20 (keeping in mind that 1 is an automatic miss).
the second thing is that DDO is an enormously powerful setting. even taking into account critical profile, the above great axe is looking at
11.025 damage per swing compared to the falchion's
9.9‡, so the great axe is clearly better... but there has never been a THF barbarian in the history of DDO with 0 Strength bonus. at level 20 you're going to have at a bare minimum 12 Strength bonus which goes into damage at *1.5 since you're THF for +18 damage, and another +16 damage from Power Attack (a feat you should definitely have). with that +34 alone we get
46.725 great axe against
47.3 falchion, and these numbers go to
51.175 and
53.75 with Improved Critical (another feat you should definitely have). depending on what enhancements you pick the gap can get really tremendous.
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it's a lot of math for sure, but it really just comes down to the premise that there are huge damage bonuses in DDO relative to base damage dice. a spreadsheet program is very helpful to see it all for yourself, which i think is valuable, but there's also a math oriented community here so we're totally a resource you can use for any questions. good luck!
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¹technically you have to roll another attack roll to "confirm" the critical hit if your threat isn't on 20; generally this isn't a significant factor for pure melee classes.
‡this doesn't take into account glancing blow damage. THF generates glancing blows on (eventually) three of your four attack animations, and since they can't generate critical hits the higher base damage weapon always does better there. however, even in the absolute best glancing blow case you're only getting 75% * 70% = 52.5% damage on them (*19/20 because 1 is still an automatic miss), and with only a 1.5 base damage advantage that's nowhere near enough to close the gap from critical hits.