The best part about the original Serious Sam is that there was absolutely nothing serious in it!
This is just my opinion, but I really don't think first person shooters can be taken seriously at all, unless they are simulators or somewhat closer to a simulation like Arma, SWAT and the early Rainbow Six.
I see them as I saw '80s action movies, pure dumb and over the top entertainment with no other goal than being fun.

You can skip the side quests. I did t find anything in them that I have actually used yet, it’s mainly gadgets and one gimmick weapon so far (oh and chances to push either offensive stereotypes or certain political agendas).

I have got most ways through France now and to be fair the combat is still really good, but I am spotting more bugs and graphical glitches. Npcs and various scenery (like trees) most you can walkthrough, some not for some reason. I have only seen vast armies (claimed) on the entry map and that was all billboards.

There is a vast array of weapons, but they tend to fall into 2 or 3 catagories. They do feel good though, and pack a punch. My favourite so far a rifle type rocket launcher gibs more than the beegees!

In terms of the upgrades, you can safely ignore them. There really isn’t anything there worthwhile. I picked dual weapon, but it’s a pain to use. You have to left alt to get the second weapon up and the left and right mouse clicks to use it. Seeing as how you swap weapons a lot that system is inherently flawed and unusable. So that’s one tree down. Then there is melee, but if things are that close your going to die! That leaves a couple of reload speeds and damages upgrades when health low. Really not worth worrying about. Much like gadgets, by the time you scrolled through them find the one you want you are overwhelmed by enemies.

The story and characters just get worse the more you go in too.

There is a good variety of enemy though, not just recoloured but actually well thought out enemies with varying attacks and strategies. The vamps for instance have a few different attacks from anything else.

I started off thinking this is good solid 3/5 game and hopefully will be improved over time, but it’s slipped to a 2/5 now.

The main underlying premise was always serious - save Earth from Mental (Tah-Uhm) by any means necessary Yes, it does take Sam on some wonky adventures and facing over the top enemies but without that premise, I might as well just be shooting endless enemies infinitely in any random sandbox or suvival game. Even the little story that is told through Netricsa in TFE and TSE are enough for me. From fighting to the timelock until he entered the spaceship at the end of TSE, it made sense. I can't say the same about SS4.

BFE respected the story of the originals and fit into the timeline. Sad to see they pretty much dropped the ball on that.

Also, bullshit story is one of the main reasons I absolutely hate Serious Sam 2, the other being the art direction (way too cartoony).

Can skip, but you are robbing yourself of some pretty big fights and supplies. Not to mention that some weapon mods you get from them only appear several levels later if you don't do the side mission.
The open world France levels were probably the worst out of the entire game. So much empty space it's insane. And looking for secrets in that? Forget it. They really should have stayed with the classic level design. Especially because the "legion" system only appears in the prologue and the last level.
Indeed. Melee is absolutely not viable, especially on harder difficulties so the only relevant parts are the things that affect speed. Reload speed, ability to reload while sprinting, increased speed when aiming down sights. With my spare points, I went into the dual wielding but that brings your power level to insane heights, so much so that I think the game is worse for it because it upsets the balance horribly. Would have definitely been better if they skipped the skill tree entirely.
Yup. A damn shame after BFE built my hopes up. It had better story and 90% of the time, it was just Sam talking to Quinn on the radio. No pointless immortal NPCs, no overlong conversations, no cringy sidequests. Most people shit on BFE but I really like it a lot.

The fact anyone takes the story in these games at all seriously is baffling to me.

There is not much of it and it's easily ignorable if you want, but it's there. Of course the most important part is the gameplay, but I view it as a plus when a series respects the previous entries in regards to continuity and established stuff.

I took the chance, got and installed SS4 - but i do not have win10.

For those of you who are worried about compatibility, i can confirm that SS4 runs nicely on windows 7 - with no problems whatsoever.

You are missing the point the of the system requirements. Sure, it may run on XP even, but if you have any issues at all, then you will not get support as you are not running on an approved system.
So the key is, buyer takes all responsibility if buying for an OS not listed as compatible.