I run a few more addons than you guys but drastically scaled down from my usual UI, and all just display/convenience orientated, currently I run:
Allplayed (tracks /played across all my characters)
Aloft (makes healthbars smaller and tidier)
Ara Broker Guild/Friends (friendslist mouseover on my top bar - was more useful when we had iron guilds as well)
AutoDecliner (blocks all invites)
BlizzBugsSuck (sorts a few UI bugs, granted those may actually have been fixed by now but it doesn't hurt)
Broker:Currency (displays my gold/darkmoon tickets on the top bar)
BugGrabber/BugSack (picks up UI errors)
CensusPlus (monitors poulation of servers for WarcraftRealms, who I've been active with for a long time)
Chatter (tidies up chat box)
ChocolateBar (top bar with info displays)
DailyQuestCounter (shows how many dailies I've done in a day)
Factionizer (improved rep display)
FloTotemBar (shaman only, gives me dedicated buttons for my totems off the main actionbar, probably not necessary on ironman but I'm used to it on my non-iron shaman)
InFlight (flightpath timers)
Moncai's EquipCompare (compares equipment without having to hold down ctrl or whatever key it is by default)
OpenDefaultBag (puts bag back to original behaviour of opening the one you clicked on, not all of them)
QuestLevelPatch (displays quest levels rather than just colour, helps me avoid them going grey)
ScrollingCombatText (prettier version of the ingame one - I do not have any ability alerts set up on it)
SexyMap (tidier minimap)
Skada (I was curious how rubbish my dps was!
)
TargetofTargetTooltip (lets me see from mouseover if someone's looking at me lol)
Wowhead looter (data gatherer)
Oof, that's more than I realised, most of them are really small and not that obvious when you're used to them - I don't believe any give a combat advantage though, aside from making information more easily visible. I do not have NPCScan enabled on this character as I feel that is an advantage in knowing where to look for extra exp (Deadster's chest info addon is probably not much different though, so perhaps I'm being overly strict by not using NPScan!), likewise I don't set up alerts for abilities, and don't use quest helper type addons (the default one is plenty anyway)