As long as you know exactly what you want, verbalizing the precise name isn't necessary (up to and incluing omitting the subject at all). Ref: GoF, Harry vs. dictionary; GoF, Mrs.Weasley vs WWW products
No line of sight needed. Ref: OotP, twins vs broomstics.
No need to know the location Ref: HP, Hermione vs Horcrux books.
No known distance limits (depends on concentration), Hogwarts to grounds is OK Ref: GoF, Harry vs broom
SQL query where clause: possible Ref: DH, Hermione vs Horcrux books.
Specificity of Item: With Accio, you don't always need to know what it is you're summoning, apparently. I mentioned in my answer regarding Hermione and Accio that the first instance of Accio that I could find is Mrs Weasley stripping the twins of their WWW products (in this case, ton-tongue toffees). She doesn't specify what she's Accio-ing but just says "Accio! Accio! Accio!" and confiscates the twins' items. (Goblet of Fire - The Portkey)
Also in Goblet of Fire, Hermione and Harry demonstrate that one doesn't have to exactly name the item being summoned.
‘Well, now we know what to do next time I can’t manage a spell,’ Harry said, throwing a Rune Dictionary back to Hermione, so he could try again, ‘threaten me with a dragon. Right ...’ He raised his wand once more. ‘Accio Dictionary!’
The heavy book soared out of Hermione’s hand, flew across the room, and Harry caught it.
Goblet of Fire - page 303 - Bloomsbury - chapter 20, The First Task
Harry did not say, "Accio Rune Dictionary!" I'm surprised he wasn't pelted by a variety of dictionaries that happened to be around when he practiced this time.
Distance and Line of Sight: When Harry attempts to Accio Hagrid from the ground when Hagrid falls out of the motorbike, he is unsuccessful and later finds Hagrid lying spreadeagled on the ground after the motorbike crashes down from the sky. Harry was not able to see Hagrid when he performed Accio after Hagrid launches himself from the motorbike onto one of the Death Eaters' brooms mid-air and subsequently falls from Harry's sight. (Deathly Hallows - The Seven Potters) As a sidenote, it is possible to Accio living creatures; Harry Accios his toad in Potions in Order of the Phoenix (chapter 18, Dumbledore's Army).
However, Harry Accios his Firebolt from his room in Gryffindor Tower during the First Task of the Triwizard Tournament, sight unseen, from an ample distance. (Goblet of Fire - The First Task) Hermione seems to confirm that distance and line of sight is not an issue as long as the person casting the spell is concentrating fully. Concentration seems to trump distance and line of sight:
‘Harry, I really think you’ve got it!’ said Hermione, delightedly.
‘Just as long as it works tomorrow,’ Harry said. ‘The Firebolt’s going to be much further away than the stuff in here, it’s going to be in the castle, and I’m going to be out there in the grounds ...’
‘That doesn’t matter,’ said Hermione firmly. ‘Just as long as you’re concentrating really, really hard on it, it’ll come.’
Goblet of Fire - page 303 - Bloomsbury - chapter 20, The First Task
Other Actively Magic Items/Accio-ing Any Item: Harry successfully Accios the Triwizard Cup as he's clutching Cedric Diggory's body, while the cup was still activated as a Portkey. I thought it was interesting that Harry merely said, "Accio!" and not "Accio Portkey!" or "Accio cup!" (Goblet of Fire - Priori Incantatum) This reinforces Mrs Weasley's use of Accio under the Specificity of Items section.
On the other hand, Accio doesn't work on Horcruxes, Harry's Invisibility Cloak (which is a Hallow, so presumably Accio might not work on the Resurrection Stone or the Elder Wand), or the Sword of Gryffindor. It seems there are items that are so magically powerful that a simple summoning charm doesn't affect them. In the Horcrux cave where the locket is hidden, Accio causes one of the Inferi to jump from the water, but then it splashes back down into the lake. Dumbledore believed that the Inferi's reaction was intuitive toward anyone who tried to take the Horcrux.
where : Yes, it is possible to summon books with a keyword, such as 'Horcrux'.
‘I didn’t think there were any books on Horcruxes in the library?’
‘There weren’t,’ said Hermione, who had turned pink. ‘Dumbledore removed them all, but he – he didn’t destroy them.’
Ron sat up straight, wide-eyed. ‘How in the name of Merlin’s pants have you managed to get
your hands on those Horcrux books?’
‘It – it wasn’t stealing!’ said Hermione, looking from Harry to Ron with a kind of desperation. ‘They were still library books, even if Dumbledore had taken them off the shelves. Anyway, if he really didn’t want anyone to get at them, I’m sure he would have
made it much harder to –’
‘Get to the point!’ said Ron.
‘Well ... it was easy,’ said Hermione in a small voice. ‘I just did a Summoning Charm. You know – accio. And – they zoomed out of Dumbledore’s study window right into the girls’ dormitory.’
Deathly Hallows - page 88 - Bloomsbury - chapter 6, The Ghoul in Pyjamas
Another example from canon is Harry summoning brooms in Hogsmeade by saying, "Accio Rosmerta's brooms!" (Half-Blood Prince - chapter 27, The Lightning-Struck Tower)
Vague Items: Could a person Accio a "meal"? I think probably. But they may not know what they'll be getting if they merely say, "Accio, meal!" However, if concentration is a big factor, perhaps they could Accio a meal of their choice as long as it's already prepared. Gamp's Five Laws of Elemental Transfiguration specifically states that food cannot be conjured from thin air. It can be conjured if it's already prepared (for example, the food at Hogwarts appearing on the students' plates is conjured (Transfigured) from the kitchens to the plates in the Great Hall). So if a plate of turkey, mashed potatoes, gravy, and green beans is in the kitchen, it can be Accio'ed. If it's not there, then no dice.
So, Accio socks? Sure. They might not be one's favorite pair, though. Accio pillow? It might not be the one you were hoping for (you know...the extra squashy one with the Slytherin pillowcase...)
Accio-able Items: You mention water. The basic spell to summon water is Aguamenti, BUT I think if it were bottled water and one said, "Accio, water!" then the bottled water could be summoned. In Order of the Phoenix, Sirius summons everyone their own bottle of Butterbeer by saying, "Accio, Butterbeer!" Dirt? I don't see why not. Gases? Is a memory in a phial a gas? If it is some sort of gas, then, again, I don't see why not. I think it would depend on the gas in question.
Rules About Accio Inferred: I think the biggest thing to note about Accio is that it is not always necessary to name the item you're summoning as you cast the spell. "Accio!" seems to be sufficient for many, many things. It seems the more dangerous or rare the item, the harder it is to Accio, even impossible in some circumstances (Horcruxes, Hallows, etc.). Otherwise, Accio seems to be a basically straightforward spell.