First part of the spell covers that. Can't speak for other folks, but I'd rule the glyph never triggers if the effect won't be harmful.
That's reasonable, but what about spells that are ambiguously harmful? Polymorph meets the qualifications for GoW (at higher levels), but could be positive or negative (obviously it would only be positive against certain monsters... or people that really, really like dolphins).
I think what's more important to note about GoW is that it has no range limit. The spell describes that the trigger for both the "surface" and "within an object" glyphs can be "approaching within a certain distance of the glyph." It has no limit on this. For the Explosive Runes, the effect is centered on the glyph itself; no big deal. For the Spell Glyph, though, the description says "If the spell has a target, it targets the creature that triggered the glyph. If the spell affects an area, the area is centered on that creature. If the spell summon hostile creatures or harmful objects or traps, they appear as close as possible to the intruder and attack it." Combine this with the range and precision of the trigger for the glyph, the fact that it can trigger up to 8th level spells (PC classes thus far only allow for 1 8th and 9th level spell to be castable at once, so far as I know, so 9th level spells are out of the question for now), and the fact that it lasts until dispelled or triggered and to its maximum duration of the spell, and there's a lot of variety available.
Now, I know that GoW is meant to be used for traps, but the possibility here is immense. If someone has the patience to keep setting up Glyphs with interesting triggers, you have an endless variety of traps and buffs. For example, let's say you're party is well aware that some specific powerful creature (Balor, etc.) is going to be assaulting whatever plane you're on. Oh, man, how ever can we prepare to defend ourselves? How about a GoW set to be triggered by that specific creature (add as much detail as you need to solidify that it will only target your enemy) when it approaches within 10,000,000,000 miles that triggers an inverted Magic Circle, and a second that triggers a Planar Binding spell when the first Glyph is triggered.
Eventually, your Balor assaults your plane, and immediately must attempt to avoid being put under your command.
OP pointed out the potential for this spell to maintain concentration on spells for you. I hadn't even considered the potential of GoW for buff spells, but when you consider, again the available triggers, you have a monster. The spell specifies that a password can allow creatures to bypass the spell, which implies, as I read it, that it could be triggered by a password. So, you set a bunch of glyphs with triggers that are set to you or allies saying specific words or phrases, and there's no limit to the buffs one could trigger with a one-word utterance. This is where I start making my wizard that triggers six GoW's that all trigger the six Enhance Ability options (Bull's Strength, Bear's Endurance, etc.) when he shouts "SHAZAM!" And as I wrote that, I realized that a seventh Glyph could be set to Thunderwave... Oh boy, I'm excited now!
Similarly, you could describe some enemy of yours, or even just some type of foe (e.g., evil Drow), that says a specific word or phrase. Trick your foe into saying the word and.... baboom!
