The generally accepted thing since the last issue came out is that once you have SOs, have no more than three of one type in a power. Any more will give you a pretty negligible effect. So, for your forcefields, you should probably slot three defence buffs in each power.
In powers that have an accuracy check to hit (like attacks, and some other Defender primary powers - there are a good few in the Kinetics set for example), you should slot one accuracy. That changes if you're planning on using the character mainly in the PvP zones, as the base accuracy is lower there - a lot of people are putting two or even three accuracy enhancements in powers for those zones.
Clicky
Go and check for the Defence buff ranges on there.
It talks all about ED if you go a bit down.
I'm still 6 Slotting mine, although I know it's a piddly bonus I'm getting, I just can't help myself.
A general guide is.
2 = No Reduction
3 = Small Reduction
4-6 = Major Reduction
That's life with ED. 
Its not so bad for most defenders I think. Certainly not horifying for Rad defenders (though I miss my 5 hold durations in EMP) because most powers take more than one enhancement so you can split it 3 and 3.
I do feel sorry for FFs and the defence only powers, but only if they feel sorry for my SR scrapper
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
My FF/Dark used to have all her bubbles 6 slotted, so when I respeced I had a ton of slots to throw about. Also I dropped a couple of powers I didn't feel I needed anymore and had spare slots from pool powers that used to have more than 3 slots in.
So now I have my bubbles 3 slotted and with all the spares I've been slotting most blasts for 3 damage and 2 accuracy. I have moonbeam with 2 dam 2 acc and 2 recharge, I like the power so I find having it come around a bit more often helps.
Also Cynic, I feel sorry for you SR scrapper. 
Since ED, I have made my own policy to use a maximum of 4 of the same enhancement type in each power. I don't think it is worth going more than this.
Try this:
US Defender Guide
Many of these have been updated to include the i6 updates.
BTW I too feel sorry for the SR scrapper.
@Byrne
Debtmeister General of The Plastic Army & Empire
I wonder if anyone could point me in the direction of a good, up-to-date guide for slotting the various defender power sets, or just leave some thoughts to consider. A few things to consider
- I don't (necessarily) want a build, just to know more about how slotting works
- I'm new to CoH and I gather that slotting powers has changed somewhat recently - I have seen a lot of talk of 6-slotting powers all with the same effect, but I think that is now out-of-date, correct?
Well, thanks for any feedback. I really like the defender AT, as I like the play style, buffing others, TP'ing out your colleagues when they are about to go under and so on. For the record I'm playing Dark Corridor who is a FF/Dark Defender, and she's really set up just for team play - all the protective fields, recall friend, grant invisibility and so on. Hope to see you around (on Defiant)!