What do you think about these european theme parks?

What exactly do you need to know? I have been to most of them and can provide further details but here is the TLDR

DLRP - Well, basically when this property was built it tanked for a variety of reasons. They added a few attractions to try and prop up attendance and it got financially bailed out by a Saudi Prince (not making this up) and just kind of treaded water until 2017 when the Walt Disney Company bought out the whole property from its investors.
While they do put some more effort into keeping the park up than they used to, and have spent a lot of money over the previous decade refurbing every attraction, it still suffers from a lack of investment into new attractions at least at the main park. I couldn't even tell you when the last *new* major attraction that park got was.
The other park (the ripoff MGM Studios) was cheaply built in order to satisfy some land use agreements they made with the government and is unquestionably the worst Disney park. They have admitted this and committed to fixing it, and in true Disney fashion that was literally 20 years ago and the park is still a mess. Avengers had its budget cut and isn't that good, from what I understand Star Wars was cancelled, Maybe once Frozen opens it will be a bit better but from what I can tell it will probably still be the worst Disney Park.
Overall you can have fun at DLRP but it doesn't get the same level of investment that the American parks do so if you are used to that you won't be impressed.

Europa Park - this park is huge and its family owned, so it is well taken care of and generally has a very good reputation (its actually owned by the same family that owns Mach Rides). It has many attractions that would best be classified as "good" although IMO lacks a real standout ride that puts it on the map. The Mach family also owns and operates the onsite hotels around the land as part of the Europa Park resort so it does kind of have that Disney-escapism aspect to it (and the hotels are actually really nice and well themed). The themed areas are all different countries in Europe and are nicely done, i'd definitely reccomend a visit.
Side note it also has a structure in the France section that looks like a ripoff of Spaceship Earth at Epcot, and it has a can-can dance themed coaster in it (I think used to be space themed).

Efeteling - basically if Europe had its own version of Disneyland that was not owned by Disney (an American company that cartoonized European culture), this would be it. It is a quirky park with a lot of dark rides and just feels very old school European.

Tivoli Gardens- Although it actually is a full fledged amusement park, it feels more like a city park with some rides, which is a good thing. It is really beautiful and probably the most famous tourist spot in Copenhagen. Much like Efteling its very unique and different from the over-commercialized larger parks. But you're never going to find big coasters here that's not what they do.

Port Aventura - well, unlike Efteling and Tivoli, this is a big corporate mega park (including neighboring hotels). Its hard to keep track of who owns this, it was created as a joint venture between Tussaud's (Merlin before Merlin) and Busch Theme Parks (Busch/Sea World). Then it was sold to Universal and then sold to private equity. So it has influences from all the above and you can tell when visiting.
The Ferrari park next to it is a total cash grab, it could have easily been a land inside the park but instead built a whole other park so they could sell another admission ticket. Personally I found the place tacky and distasteful, it has some Italian looking buildings, a couple bad dark rides, S&S double towers, and little car ride, some kids flats, and of course the Intamin accelerator coaster that gets all the thrillseekers to buy a ticket. They put the bare minimum amount of everything to ensure people buy a ticket but the place just feels very generic, bland, and cash-grabby to me (not that I hate it, but I would have liked it if they had just built a scaled down version inside the park not a whole other park with a whole other admission).
Overall PA is a solid park although its operations can be hit or miss if that kind of thing bothers you (it does me). Shambahla is probably my favorite coaster in Europe.

Wow, thank you so much for this detailed answer!!

I have already analyzed reviews on TripAdvisor, and the majority of the responses were similar to yours. I haven't been to these parks, except Europa-Park, which simply amazed me. I adore every little thing in that place!

Although it is saddening in the case of Disney that a park with such a huge fan base isn't being properly taken care of.