Enhancement Diversification is a system they brought in some time ago to stop people slotting any one aspect of a power too strongly. Its effect is to gradually reduce the benefit you get as you slot more and more of the same aspect, until once you're over a certain threshold you get very little benefit at all if you try to add further boosts.
For your Force Field power the critical threshold is 60% (as defence buff's a Schedule B power - see the table on the linked page for details) - as a general rule it's OK to slot up to, and slightly over (as the odds are the last enhancement you add while under the threshold will take you over it), but once there you might as well stop trying to go any higher. Basically if you're using Mid's it's OK to slot until you get that high rate warning.
One thing worth baring in mind is that while boosts that come directly from enhancements always count when ED's being worked out (even if you're mixing and matching SOs, common IOs, set IOs, etc...), boosts from set bonuses are separate from ED. If standard slotting from enhs gives you 61% and you add a set IO to get a +3% DEF bonus, you get the full +3%.
Hope this helps!
Read this:
http://cityofheroes.wikia.com/wiki/E...iversification
The short story is that it's generally not worth slotting more than 3 of the same type of SO/Common IO in a power. Set IOs complicate matters tho 
Octavian Vanguard
@ohmsEU and @ohms 2
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For your Force Field power the critical threshold is 60% (as defence buff's a Schedule B power - see the table on the linked page for details) - as a general rule it's OK to slot up to, and slightly over (as the odds are the last enhancement you add while under the threshold will take you over it), but once there you might as well stop trying to go any higher. Basically if you're using Mid's it's OK to slot until you get that high rate warning.
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So, what you're saying is, what I'm doing is ok, but I should probably stop where I am? Just trying to give my MM the best amount of defense possible.
Well, you don't absolutely have to stop, but you'll get very little extra if you try to slot PFF's defence any higher; add an extra enh that would notionally give you +20%, and you'll only get an actual increase of +3% (15% of the normal effect).
In most circumstances you'll be able make much more effective use of that extra slot on some other power where ED isn't yet an issue, but if there is no such power and getting the very best from PFF's important, you might just want to put more +DEF in PFF anyway.
Or, check and see if you could use that slot to get extra +DEF from an IO set bonus - the numbers tend to be a bit small there, but they aren't reduced by ED.
Been using Mids Hero Designer v1.4 for a while, trying to get my MM perfect.
But when I stuck several defense IO's in for example, Personal Force Field, it would then say somewhere that a high-rate of ED Reduction has been applied.
Typical Message:
PERSONAL FORCE FIELD
Defense:
Total Effect: 82.88%
With ED applied:59.43%
ED Reduction: 23.44%(28.29% of total)
The highest level of ED Reduction is being applied.
Threshold: 60%
Can someone tell me what this means? It'd be a great help.
NOTES: PFF is 4-slotted. 2 IO's from Red Fortune Set and 2 from Luck of The Gambler Set;
Red Fortune: Defense, and Def.+ Rech.
Luck of The Gambler: Defense, and Def.+ End.