The basic logic of alternate planning assumes that the approach may be flown under worst case conditions. So the 600-2 for a precision approach, the worst case conditions would assume the GS was out of service and that the localizer option would be used and that circling might be required. The 600-2 usually will meet the worst case conditions for a localizer. When it doesn't, higher alternate minimums are specified. In the case of a non WAAS GPS dependent on RAIM availability and adequate geometry of the satellites, it is only prudent that you only use GPS for your destination or your alternate, but not both. With WAAS, basic RNAV (GPS) LNAV integrity is now considered almost a sure thing and a WAAS GPS does not have the limitation requiring supplementary navigation and can be used as the sole source for satisfying 91.205 for IFR navigation.
So with a typical RNAV (GPS) approach, with a WAAS GPS, the worst case conditions are that the vertical guidance is not available or that the lateral precision may not be met for LP and you have to fly the approach using the LNAV minimums and have to circle to land. When those exceed 800-2, you will see the inverted A indicating special alternate minimums.
There are other criteria that must be met in order for an approach to be usable as an alternate, the approach lateral guidance must be monitored or the procedure will have the reverse "A" NA indicated. Also, there must be a local altimeter setting available, so you will often see the note NA when local weather not available or when the tower is closed. At my airport, Rock Hill, SC, KUZA, there is an ILS or LOC RWY 2 approach, but since the airport is not towered, the ILS is not monitored, so it can't be used as an alternate. The RNAV (GPS) RWY 2 can be used as an alternate, because WAAS is monitored at the national level. The alternate minimums are 800-2 except for Cat D, where they are set to 800 - 2 1/4 because the circle to land on this procedure requires 674 and 2 1/4 NM for category D. this approach also has the note "NA when local weather not available".
Look at the Alternate minimums for KJQF, Concord, NC and you should be able to figure out why the alternate minimums for the ILS 20 for category D are upped from 600-2 to 800-2 1/2.