In reply to andy farnell: Bit of an essay coming on, I'm going to look at this from a (semi) mathematical perspective
Climbs have many dimensions to their difficulty, the usual suspects; technical difficulty of the hardest move (or sequence), exposure, rock quality, sustainedness, obviousness of moves, amount of protection, injury/death likelehood if you come off, etc. I'm sure I've missed a few.
Anyway, the UK grades work like this: E grade is the overall difficulty (all dimensions), tech grade is the difficulty of hardest move (1 dimension). Relationship between the two tells you about all the other dimensions.
US grades: 5.nn grade is the overall difficulty, R/X is the danger dimension.
French grade: overall difficulty but usually applied to routes where some of the dimensions (e.g. quality of pro) have collapsed to zero.
Bouldering grade: overall difficulty but where several dimensions (e.g. pro, exposure...) have collapsed to zero.
Now; dual grade systems (obviously) give more information than single grade systems so the UK grades give more info than French grades. But this is because UK grades have developed from trad which has more dimensions to difficulty and so you (typically) need more info before venturing onto a route. Sport climbs have fewer dimensions (less danger) so a single grade system is sufficient to tell you enough about a route without you getting hurt.
Also, because they deal with different dimensions of difficulty, it's impossible to exactly map one grading system onto another. What Andy's suggestion provides is a way of mapping French into E grades and the other way too. If it can be applied consistently (which is the key to making any grading system effective) then it may well help those making the transition from trad to sport or sport to trad, and also those operating at the highest levels where grades are far less well defined, partly because there is less consensus of opinion (due to less ascents).
I could go on with more points, but can't be bothered.