I actually kinda agree with you on the easy part, though I found I actually had more fun when I replayed on normal. First time I went through I played on hard since I'd heard in reviews it was easy. Turned out it was fairly easy for the most part on hard, but I had to concentrate on the enemies more and use the more "Efficient" combat plasmids like electrobolt. Playing through on normal I had more freedom to do silly crap since things weren't as ridicilously durable. And some games I'd agree challenge is a big part of it. RTS games for example, generally 95% of the time I spend in an RTS is either skirmishes or doing humans VS AI multiplayer games, so challenge goes a long way there.
Bioshock though, most of my enjoyment comes from the feel of everything. As mentioned before, it's pretty as all hell, not to mention the cohesive art direction in every little bit. The sound is almost as good, the only real drawback being you hear "JEEEEEESUS LOVES ME THIS I KNOW" way too often, but even that didn't really bother me because I don't really remember a single part of Bioshock's voice acting that irritated me. Listening to the diaries, imagining how everything was before it went to shit, putting together the backstory myself instead of looking on a wiki. Then there are the even simpler things I enjoy, like the animation and sound of the shotgun (Never get the rate of fire upgrade! NEVER!), the little PSAs, just about everything.
Simplified System Shock, yeah, but again I don't think it's a bad thing. I didn't really miss inventory management, and while that would've given it a more "Survival horror"-esque feeling by limiting the ammo and weapons you could carry, which certainly would've been good, I didn't really mind it not being there. And for not having skills or stats to level up, eh.
Railroading I didn't really mind since for the most part it was relatively loose railroading, though moreso earlier on. As in "We'll let you loose in this area, run around and do things until you find what you need to do to advance", which is closer to SS2 levels of railroading than SS1's crazy go nuts wanderthon. And hell, most games railroad in some way or another, though Stalker's a recent example of pretty open FPS. Even popular and good FPS games like Half-Life 2 and Portal (Also a game I replayed. Damn 2007 had some good stuff) are pretty damn linear and railroaded. It's something I generally appreciate when it's taken out, but don't mind if it's in.
And stupidly overpowered wrench, eh. Goes into "Easy game but I don't mind" category. And speaking of that category, even though you didn't mention it I've seen people mention it elsewhere and it kind of gets my goat, but why the hell do people complain about vita chamber abuse? Yeah, the game lets you die and respawn and kill a big daddy with a pistol with no penalty. If you don't like it, it's easy as hell to avoid doing for god's sake. Just think of it as a built in IDDQD for if you're frustrated with a fight. Even before they patched in disabling vita chambers, reloading your game if you die works too. Sort of blew my mind, like people complaining you could make godly potions in Morrowind and one punch kill anyone in the game. Just because the game lets you do it doesn't mean you should, play whatever way is the most entertaining to you.
Someone needs to make a No Splicers Allowed webpage for me.
Oh yeah, and just remembered. The "Oh noez your enemy is thiz other guyz?!" is pretty much the worst part of the plot. I got most of my enjoyment out of pre-shitstorm story and listening to the various characters, a good portion of the actual "Game" part of the story is fairly meh. The game goes a bit downhill after AR cutscene due to less pre-shitstorm story, and the final boss was kind of worthless, buuuut I gotta say I enjoyed the good ending. Games rarely do happy endings, and when they do I rarely like them, but Bioshock's worked for me. Also glad it wasn't a cliffhanger or vague in any way to set it up for an easy sequel. Heard rumbling of a Bioshock 2 and I'm a bit iffy on what they'll do since it had pretty definite endings either way, but I guess a prequel might be fun.
Ugh, this is what happens when I'm not scribbling a silly BoS story, I ramble incoherantly about Bioshock at 5 AM instead.
Edit: Sweet lord, wall 'o text.