"It's Bethesda's game right? If they still can't produce software without bugs and glitches give it a fair score, not based on money they've spent on advertisement here"
Bethesda published it, but the actual team who crafted it are Arkane Studios, whose previous includes Ark Fatalis, Dark Messiah of Might and Magic and design work on BioShock 2.
Dishonored is by and large a bug-free game. Occasionally a graphical glitch will cause pinchable items on NPCs to levitate a few feet in front of them (which makes no difference in terms of functionality). However I was able to overlook that issue because, as you rightly point out, I'm massively corrupt and Bethesda bought me a Ferrari in exchange for a good review score.
"How are the save checkpoints? If I die and restart from the checkpoint, does my health and mana get refilled, or do I restart with what I had when I was last at that checkpoint?"
It uses a quicksave system. As such, your rations remain at the level they were when you saved. I haven't got the game to hand, but I think you can have around ten slots open.
"Do you see room for DLC in the game? How do the bone charms work and is there a story behind them in the context of the game?"
There's plenty of room within the narrative for DLC - much is left unresolved by the story's end.
As best as I can remember, bone charms are extracted from the bodies of whales that have been killed for their oil (which is Dunwall's fuel of choice). It's illegal to collect bone charms but a little thing like that won't stop Corvo, particularly as he can use them to upgrade his stats.
Think of them as working like Call of Duty's perks. Their effects are permanent for as long as you keep them equipped, and tend to consist of stuff such as increasing Corvo's foot speed, decreasing the amount of time he takes to choke someone out, or lengthening the amount of time you can possess a rat.
You begin with three slots, but further slots can be purchased from Piero at the Hound Pits Pub.
"Can you upgrade to bigger swords and guns? Or just upgrade the weapons you begin with? Any boss type battles?"
You get one gun, one sword and one crossbow. You can upgrade them by seeking out hidden blueprints and handing them to Piero at the mission's end. One of the first things you should do is increase the bullet capacity of your pistol as the default number is pretty stringy.
There are no boss battles per se, so Deus Ex: Human Revolution vets can breathe a sigh of relief. There are however boss battle-style situations. Targets, particularly later in the game, tend to be well protected.