Wheels for a start – mine must be a good 1/2lb+ lighter than yours (Stans Olympic on Tune hubs). FSA stuff always weighs more than claimed – I've got a Syntace F99, FRM 100g flat bars. Rebas are good forks, but not superlight – my RC39s save me another 1/2lb (and work well when they work – the only issue is the maintenance!) SDG Fomula FX you mean? A quick check suggests that's over 200g, so far from light – I have an SLR at 130g.
There you go, significantly lighter bits than all of yours – that's almost 1.5lbs less in my equivalent parts. The thing is, the cost of those parts isn't actually significantly different to the ones you have (well apart from the wheels maybe, but mine were still cheaper than ubiquitous – and heavy – Crossmax SLs). It only gets expensive when you start upgrading perfectly serviceable parts with lighter ones. To get really light you have to be pretty obsessive about every single part though. My bits are on a 21.5lb full sus – plenty of hardtail frames around which would save a couple of pounds of that. Not cheap in total, but not that silly either compared to the cost of a lot of people's bikes on here (mine was ~£4k to build, though most hardtails would be significantly cheaper).
To get to 18lbs you have to get really silly though, and use parts which aren't actually that practical IMHO.
