In general, I like boons that do the following:
1. Make new options available.
Special animal companions, archetypes, PrCs, unique items etc. It reminds me of the excitement I felt in Living Greyhawk when the ARs were passed out and you found out what you got access to this time. In many PFS scenarios I've missed the excitement of finding loot since I know it will all turn to gold at the end and the chronicle will probably contain items I can either purchase with prestige or are too expensive for me to afford.
2. Remind me of big accomplishments in my character's career.
This includes things like learning new languages or getting broad, specific bonuses against certain creatures or cultures. This is also cool when it's a double edged sword. i.e. You defeated an invasion from the plane of earth and saved the dwarves, you gain +1 natural armor and -1 to dexterity based skills as your skin hardens.
3. Provide useful charged abilities.
A charm from a priestess of Sarenrae that can be expended to heal 1d8+1 as an immediate action or a promise from a priest of Pharasma to cast Raise Dead once for free are things that will always be useful.
In general I dislike boons that do the following:
1. Require a spreadsheet to track
As characters move into higher levels they each have an ever increasing list of abilities and powers to track. Adding boons that make me try to remember if I get a +2 to this reflex save vs a dragon's breath weapon from a scenario I played a year ago or if I already expended it or if I got it with a different character frustrates me. I'm likely to just ignore such boons unless they come up often, like say social bonuses with pathfinders.
2. Require more than a minute or two of reading/explaining for a judge to understand.
Any more than that and you're starting to significantly cut into play time, especially with 6 characters and the possibility of multiple boons each.