In ETOPS, there is a difference between an adequate airfield and a suitable airfield.
An adequate airfield is appropriate for diversion on a planned ETOPS route. The assessment which goes into that ensures things like:
a) airfield equipped with the services like ATC, lighting, communications, weather reporting, navaids, safety cover, etc..;
b) at least one letdown aid must normally be available for an instrument approach
What makes an adequate airfield suitable for use as a planned alternate on an ETOPS sector is that:
at the anticipated time of use, weather reports and/or forecasts indicate that the weather conditions are very likely to be at or above the appropriate minima (either planning minima before dispatch or enroute minima once airbourne). Off the top of my head the wxr must be at or above these limits between 1hr before the earliest landing to 1hr after the latest landing might be made at each airfield.
The minima takes into account cloudbase and visibility, crosswind limits, etc.. The crew must satisfy themselves that on the day in question sufficient adequate airfields are available which are also suitable for the intended operation. This is the ultimate responsibility of the commander.
If we are on an ETOPS sector, we of course also must have a destination alternate as usual. This is a legal requirement. How many ETOPS alternates are required depends on the time limit imposed on the aircraft. With our 180 minute approved 763s we can cross the Atlantic comfortably with just two ETOPS alternates. If you were working to 120 or 138 minute rules you would need three (Keflavik comes into consideration here over the North Atlantic).
There are no additional requirements for 180 min ETOPS, generally fewer suitable alternates are needed since the aircraft is certified for flight on single engine for a greater time (= distance).
I used to love the smell of Jet-A in the morning...