An Aga. There was one at our previous house when we purchased it and we tried using it for a while but hated it so much we sold the awful thing.
Home gym and/or home cinema. Pointless.
TVs everywhere, particularly in the bedrooms. We have one - admittedly large, although not wall mounted - and that's enough imho.
Boiling water tap - I probably boil our kettle no more than once most days so don't see the point. Our kitchen tap cost a similar amount (£900), so it's not a cost issue.
I know ensuites, kitchen islands, taxidermy and hot tubs aren't popular on mumsnet, but I have no issues with ensuites personally (think they're very convenient especially when there are guests in the house) and miss ours.
We have a largish kitchen island that gets used daily which I love (had one at our four previous houses too) and we've collected antique taxidermy for 20+ years. I see no problem with creatures that died and were subsequently stuffed many years ago.
I love hot tubs too - we had one for several years and had it professionally moved twice when we moved house. However, for me it's all about luxuriating in hot water as opposed to the jets, so next house I'm planning on an outdoor bath or tank with hot water supply connected.