A real essential, is electrical power. If possible, a permanent cable routed from your distribution board, with its own MCB, 30 amps will give you ample to run a small lathe, mig/stick welder or an air compressor, of course you won’t be running all at once. When setting out your power system, try to use double outlet sockets, and install as many as possible (if bought in a packet of 10, these are quite cheap) in strategic positions, such as at the work bench, near to doors for when working outside. An external power outlet is very useful as these are weather proof and avoid running extensions out through doors or windows.
Good lighting should be made available, in one shed all I had were a few paraffin storm lamps, it was almost impossible to see what I was doing. Ceiling mounted strip lights provide good all round, diffused light. For concentrated areas, an angle poise is good. Inspection lamps enable light to be taken to the darkest corners of your shed, where all the huge spiders hang out. Mirrors will allow you to see behind frame rails and panels that otherwise would require major stripdown to access.
Exterior lighting is very useful when you are just doing a spot of fettling, having to work outside during the dark hours or when friends have called for a "shedology meet". Exterior lighting can be provided by bulkhead fittings, these are weatherproof and very cheap from "Ye Olde DIY shoppe", for serious lighting, a 150 watt flood will bring daylight to the job.
Usually in a shed, space is of a premium, this being so, maintaining a clear working area can be very difficult. Prior to starting a job, try to give yourself as much space as you possibly can. This often requires that some equipment must be taken out and made as secure as possible for the duration of the task. When work is required to be suspended, try to take time to clear away any tools and equipment that are finished with. Assemblies like wheel spindles and spacers should be re-assembled, as should nuts to their parent bolts, to avoid inadvertent loss.
A good sturdy work bench is essential for inspection of components, the dismantling and reassembly of complex components. If possible a stout vice, bench grinder and drill press are important additions to a workbench. Around the sides and far edge of the bench, it is a good idea to have a "save all", a rail that will prevent small items from disappearing into the unknown by rolling off the bench.
Always have a good supply of rags handy in your shed, old t-shirts are good material, old towels, I have found are best. Spend some time cutting them into usable sizes, about 30cm (1ft) square is about right for most tasks. I keep mine in a large plastic flip-top bin, so that they are readily to hand. If they are not too badly soiled, they can be washed and re-used, wash them with your coveralls, and wash them twice, to flush the washing machine (unless you have a dedicated washing machine). If heavily contaminated, or swarf ridden, throw them out, they are, after all, only rags!
Shed security is always a prime concern, with so much equipment and so many tools, not to mention your prized motorcycles, there is an awful lot of kit in the average shed. It is important that the shed is locked up, even if you are leaving it for only a few minutes. If you have windows for natural light, make sure the are barred, and obscured so the rabscallions cannot see what lies within. Always positively vet all visitors to the shed, and make forced entry as difficult and noisy as possible. Alarm systems can be rigged up quite easily to run off an independant 12 volt battery supply.