Sure, here's a comprehensive list. I've gone through multiple resources on the internet to make sure I cover everything, but if I'm missing something let me know!
(Also, you may consider some of these changes spoilers, so don't read on if you don't want to know them! I have stayed clear of any plot spoilers and anything else that is not a change with game mechanics.)
Major changes
- Movepool changes (credit Smogon): various Pokemon from previous generations have gained access to new moves, though TM or by an addition to a level up or egg move list. Some, such as Mantine gaining Roost and Froslass gaining Will-O-Wisp, are fairly substantial buffs for them in competitive play.
- Base stat and ability changes (credit Kaphotics): similar to the release of X/Y, several Pokemon received base stat and ability changes in this new generation. The most notable are Gigalith, Vanilluxe and Pelipper gaining weather-inducing abilities. (This list is missing a buff to Mega Alakazam, which now has base 105 Special Defence making its base stat boost on par with other Mega forms.)
Other changes
- Thunder Wave had its accuracy reduced to 90% from 100%, which should hopefully make it more manageable.
- Dark Void had its accuracy reduced to 50% from 80%, and can now only be used by Darkrai (fails on Smeargle). The move will be virtually useless in competitive play now.
- Gale Wings, as you mentioned, now only works if the bearer is at full HP. Talonflame and Fletchinder will suffer in competitive play especially.
- Sucker Punch has been reduced to 70 base power, from 80. Perhaps it offered too much control.
- Any move under the effects of Prankster will now fail if used on Dark-type Pokemon. This is jointly a nerf to the ability and a buff to Dark-type Pokemon.
- Confusion (status condition) now only triggers 33% of the time, down from 50%. Hopefully it won't be as irritating to manage now.
- Parental Bond's second hit now deals 25% of the original damage, down from 50%. Mega Kangaskhan will hopefully be more balanced now.
- Leech Life, as you mentioned, has been increased from 20 base power to 80. This is pretty cool for a number of Pokemon, even if Bug isn't a very good offensive type for extra coverage.
- Flying Press has been increased to 100 base power. Hawlucha will appreciate this.
- Paralysis now reduces a Pokemon's Speed to 50% of its original value instead of 25%. This is yet another nerf to Thunder Wave also.
- Burns now reduce health by 6.25% per turn instead of 12.5% (1/16 instead of 1/8 of health).
- Turn order is now calculated after Mega Evolution instead before in previous generations. This indirectly nerfs Mega Sableye, who no longer gets to use Prankster on the turn it evolves, and buffs others like Mega Lopunny or Mega Beedrill that now get their Speed immediately.
- Soul Dew now boosts Psychic- and Dragon-type moves used by Latias and Latios by 20%, instead of its previous effect.
- Misty Terrain now blocks confusion, another small nerf to the condition that may have more relevance with Tapu Lele's popularity.
- Repeated use of Destiny Bond will now increase the chance that it will fail, which was previously always 0.
- Tackle is down to 40 base power from 50.
- Swagger is now 85% accurate, down from 90%. Another nerf to confusion!
- Ally Switch now has +2 priority instead of +1, which will allow it to consistently move before common priority moves that aren't Extreme Speed.
- Fell Stinger now has 50 base power, up from 30, and increases Attack by three stages instead of two. This is a cool move that received little attention in Gen 6 that might have some use now.
- Diamond Storm's 50% chance to boost its user's Defence by 1 stage has been buffed to 2 stages. This combined with the Mega Evolution change will make Mega Diancie very scary.
- Parabolic Charge has been increased to 65 base power, up from 50. This will likely remain unpopular.
- Water Shuriken is now a Special move, and receives an increase in power when used by Ash Greninja.
- Mystical Fire now has 75 base power, up from 65. Delphox might find more use for this move now.
- Normalize, Pixilate, Aerilate and Refrigerate now boost Normal-, Fairy-, Flying- and Ice-type moves by 120% respectively, down from 130% in all cases except Normalize which never boosted power previously. Galvanize, a similar new ability, is also subject to the same 120% boost.
- Sheer Cold now fails if used against Ice-type Pokemon, and has a reduced chance to hit if the user is not an Ice-type. Other OHKO moves have not been affected.
- Weak Armor now boosts Speed by two stages instead of one, hopefully making it more advantageous to use.
- Sleep and freeze now increase catch rate by 250%, up from 200%. Burn, poison and paralysis remain at 150%.
- Super Potion and Hyper Potion now heal 60 HP and 120 HP respectively. This means Super Potion was increased 10 HP and Hyper Potion was decreased by 80 -- but now also costs significantly less.
- Fresh Water, Soda Pop and Lemonade have been nerfed, previously healing 50 HP and now down to 30, 60 down to 50 and 80 down to 70 respectively.
Theories and rumours that Unaware, Scald, Knock Off and Eviolite have been nerfed are all untrue.
