When I read the title, I thought 'No, who on earth would want to go to Europe in (their) summer? Expensive, horrible crowds and too much competition for attraction tickets.
The I read the article
@Mattg and the sense in which you wrote it. I'm always a shoulder season visitor.
Agree with you (and Mattg) as broadly the best times to visit are September-October and second best, perhaps mid March to very early May. With the latter, Easter and school holidays may complicate the fares situation.
In winter, Mattg wrote how it's 'cold' (in northern Europe) but omitted how one can often have wonderful sunny days with snow having previously fallen and extant. With a reasonable, perhaps hooded, parka/zipped jacket and if so inclined, thermal underwear, one can maintain a comfortable temperature. I am not a skier but as long as one avoids icy parts of footpaths (stupidly once fell), it's good.
Mattg doesn't give the impression he had a Eurailpass (also valid in the UK) or BritRail pass, but if one does, even summer travel is achievable with the caveat that in nations like France, compulsory-booked TGVs are 'quota controlled' for passholders (as IIRC is Eurostar across the Channel) so book seats early is the mantra. See the 'seat 61' and 'Eurail' sites for more information: the latter offers reservations once you have your paper or electronic Eurailpass.
Mattg also referred to how 'some' European hotels lack aircon. Perhaps he doesn't stay in 2.5 to 3 and occasionally 3.5 or less frequently 4 star as do I, but a majority of European (and Pommy) hotels lack airconditioning. It can be stifling in many during summer as they're also poorly ventilated: having experienced that once, I'm not keen for a repeat.
And who needs to redeem constantly devaluing FF points (especially with substandard Qantas) when inventively using second tier airlines and changing in Asia, or sometimes travelling striaght through on the one airline can deliver return J fares for under A$6500 even with the state of the A$?