The large US airports have enough slots that competition is mostly limited to those who want to provide it.
The really large European airports very much don't.
Gameplay is different, depending on where your HQ is, how much competition you have, etc. Start in ORD with only one other proper airline, and you just want to expand quickly, and know that slots won't be much of an issue. Start in CDG with 5 others, and you'll find yourself expanding less optimally in order to make sure you get slots.
Agree it gets very boring when you have HQ to yourself, and all the slots are gone. But think it gets almost as boring when you have your HQ to yourself, you fly practically every viable route, and there are still 20+ spare slots in most hours if someone wants to turn up and eat into your profits.
I like the challenge of dealing with limited slots as your airline grows. Like in JA, where NRT was very full, very early, and getting more planes meant turning SH flights into LH, to make more money per slot. So lots more slots early, to make expanding in CDG/LHR/etc the same as expanding in ATL/ORD/DFW is something I wouldn't want. More slots later in the game, so that 15 years in sees ATL being similar to AMS could help keep interest longer.
My main gripe is with how slot releases are dealt with. That expanding more efficiently than your direct competitors becomes much less relevant than just being online at the right time. That's not just related to slot numbers vs demand, that's also related to plane prices, profits, number of airlines in your HQ vs start-up cash. In the Euro challenge, it's possible for expanding more efficiently to make a big difference, it will be tough right now for an airline to instantly grab all 168 extra slots when they appear, just because they're online first. a 2:1 slot lead during that initial expansion won't suddenly disappear. Whereas in JA at NRT, it was trivial to do it, both airlines were rolling in enough cash to grab all the slots they had access to, a 2:1 slot lead quickly turned into a 1:2 deficit, purely due to being online at the wrong times.