I have done this a few times for various types of job, including bar and counter tops.
To begin with it is important to reconsider how to use cement as a fine material rather than a building material. First thing is to understand that cement needs water to cure, lots of it, if it dries it will be a dusty weak mix.
for most building work this isn't as critical as for something like a worktop.
So when you make the mold ensure that is is made of a non-absorbent material - i have used plastic and glass for molds, (which both incidentaly give you a very high quality surface finish) and have always added a cover of polythene to keep the moisture in, while the slab cures.
As you can imagine it is best to cast the slab upside down, so the cast top surface, which is most likely to be poor, ends up underneath.
Another thing to ensure a good dense slab is to vibrate the mix for a while as you pour, and for a while afterwards - leave it to your imagination for what to use for vibration.
The mix itself needs to be different from a building mix - you can use sand, but make it clean stuff and bear in mind that its colour will show, better still use crushed stone to suit, limestone slate etc. As for the type of cement, OPC is good, especially the white one, even if you need to get a dark colour with pigment (plain OPC makes everything have a greenish tinge) - I particularly like to use ferrocement (Ciment Fondu) it has a great neutral dark grey colour and cures quickly (you will need to allow time for a full cure, not just a set, otherwise it will powder).
Rethink the idea of reinforcement, steel is too cumbersome for this, unless you buy some fine mesh (no point in using Stainless). The best reinforcement is to use chopped strand mat, the stuff you use for making glass fibre, use it to replace the sand content of the mix, and work a cement slurry well into successive layers of the stuff, leaving as little air as possible. this way you can make a very thin, very strong slab of concrete (perhaps the word concrete is wrong here). I have seen a canoe made from CSM with ferrocement, with walls only a few mm's thick.
Of course making a slab with so much glass fibre in it doesnt allow you to get very decorative - so make a composite slab; make the mold, add a cm of surface layer (decorative finish) to the surfaces leaving a space for the CSM core mix.
As for decorative, there are a load of different additives you could try, pigments, natural agregates, glass, fibre-optics, use your imagination. If you add any inclusions or agregates it is worth thinking about grinding the top surface down to expose the quality of the inclusions, you can do this with a set of diamond grinding discs (hire them) just like you would use for terrazo - I once saw a casting that had old engine parts, cogs and gears laid into it, then it was ground down to expose their sections.
I hope that gives you something to work with - best tip is make a small practise slab first, something paving stone sized, to get your hand in.
