I've been pondering the same thing over the last few days as I was thinking about building vs. buying. So far it seems that for me, it would have fans, ground space to grow as well as table space to grow, some sort of heat source (passive solar?) for the cold weather and automatic window vents that will open and close when temps get too hot. It would also have a drip irrigation throughout. This is only my second year so i can only imagine what I would type if you asked this again next year LOL

I installed an automatic window vent on the roof ridge of my 10x16 glass greenhouse and found out quickly that it just wasn't enough to combat the heat of the sun. A few years later, I installed an exhaust fan on a thermostat and it works so well, I should have just done that from the beginning.

I think the ideal hobby greenhouse would have electricity, running water, and exhaust fan and heater on thermostats. A potting bench area with cupboard storage for nursery pots, fertilizers etc, plant benches, heating mats for propagation. Depending on what you want to grow, you would ideally have a spot for in-ground growing and benches for pots and nursery flats. And it would be as large as you could possibly have room for and afford! I've never seen anyone complain their greenhouse was too large. :-D

To answer that question we'd need to know your climate, the crops you want to grow, and your objectives. The ideal greenhouse would be built to grow what you want in the best manner possible to meet your objectives. You might even be more interested in appearance than crop results.

In short we need more information.

air conditioning and heating if money is no object.

Other wise automation of natural ventilation and heating is pretty much mandatory.

As wyndyacre said, ventilation needs to be one of the primary concerns. In my 10x16' stick-built greenhouse, over-heating is more of a concern than keeping it warm when it's cold outside. Thermal mass can help: gravel/concrete pavers for the flooring and barrels of water where there might be space. (Those plastic jugs kitty litter comes in work well.)

I've got water and power: installed a 220v sub-panel that was needed for the adjacent hot tub. Plan on heat...as well as a back-up in case the power goes out. Electric resistance heat is the easiest to install but the most costly to use. Only recently I installed a 220 v circulating heater I bought at Northern Tool. ($249 at some places, $149 on the web, $99 on sale last December....) I modified it so that just the fan can be used in the summer months.

Consider the purpose of the structure: is it just for plants or do you plan on chillin' out there in the winter? How about a mini-fridge and tunes? There are some inexpensive sound systems with an I-pod dock,,,, Of course, most greenhouses are like storage sheds: no matter how big you build, it will only be half-big enough....

Cheers

I like that last part. Mine is 27' x 66' and it's not big enough for me.

I'm in Texas and with the ends open it still gets too hot during the day inside the hoophouse and cools off too much at night. I have an aquaponics system with over 5,000 gallons of water in an aquaponics system, using propane water heater as needed to keep water over 60.

That responses have given you some ideas to ponder but Devoldling mentioned automation of ventilation and the key word is automation. Now if you want to pay for it you never need to leave the comfort of your house to keep everything ideal within your greenhouse. Thermostatic controls can run fans, open louvers, rollup sides, turn on propane or electric heaters, and just about anything else. Timers can control watering devices. Fertigation devices can inject precise fertilizer solutions into the water supply.

Depending on your situation you could even look into swamp coolers and motorized shade covering. I'm certain that you could set all these devices to take actions you initiate from hundreds of miles away using your cell phone. You might have to define IDEAL as conservative optimal or Bill Gates optimal.

Depending on the climate and your personal schedule.

If you have to leave for work early, it may be too cold out still to open up the greenhouse and by lunch it's already too hot.

If your schedule is later you may not be there to close it up on time in the evening. Overcast days it needs to close earlier than on sunny days. Less critical if you're heating it but will cost you more money.

By automated I mean as simple as vent motors on thermostatic controls. Fans depending on your set up.

Maybe my situation is just odd, I don't know.