Alpine packs: Things to consider for your alpine day pack
First, capacity. You need at absolute minimum 30L. Xieheng goes with 35L, smallest in the team. I went with 32L before, which is fine for day-climbs, but becomes uncomfortable trying to haul for multi-day trips. And you end up with way too many things strapped on outside... A 40-50L that can compress down (to 30-ish L) is a great, versatile choice. Look for compression straps on the side, which can also be used to strap on snow picket/tent/thermarest/ trekking poles/etc.
Second, pack style. Most alpine pack are top loaders. Simplest construction, which minimizes number of seams, improving water resistance and durability, and reduces weight. Limits accessibility to whatever is at the bottom, so requires good packing. Panel loaders with zips are rare, more common on ski packs, but the accessibility is nice. Most top loaders have a floating lid, which may be removable; can store small items here, or remove the whole thing for more weight-saving. Placing ropes over main compartment and securing it under the lid is a good way to carry ropes. Roll-tops becoming pretty popular too, better water resistance and rolling down the top allows for more compression, but open/closing gets a little tedious and storing small items can be troublesome. Find what works for you. Oh, having a haul loop at the front of the pack is a nice feature that helps when you have to shift the pack around.
Third, ice axe and crampons. You'll need ice axe/tool attachment points. Lots of options, traditional straps, clips or eyelets. Ideally something that also shields the pick points would be great. Cilogear's system is considered by many to be the best, but whatever suits ya fancy, long as it's secure. Ya can either stow ya crampons inside ya pack (Xieheng's style, need a crampon pouch) or strap to the outside. A lot of packs now have a reinforced patch on the front and straps to secure crampons. Storing inside ya pack is more secure, but takes up space. Jason's Alpinisto pack and Ernest's Direttissima pack have a foldout crampon pouch on the front, another option, but adds more weight than is really necessary.
Fourth, waist belt. Nice to have a waist belt when hauling a heavier load, but must achieve a delicate balance. Too thick and it will get in the way of your harness, too thin and it won't support the weight. Extra gear loops on the belt can be useful. For day climbs, waist belts aren't really necessary, Xieheng doesn't have a waist belt on his day pack at all. Could get a pack with removable belt. Most of us just let the belt hang loose (too troublesome to remove).
Fifth, support system. Usually consist of a foam pad, some packs also have aluminum stays or a pre-shaped plastic pad, usually removable. Won't be as supportive as a large pack, but enough for loads up to 15kg. Can even remove the support for day climbs, just pack properly. Some packs have a foam pad that unfolds and serves as a small bivy pad. For our climbs, will be important that the day pack support is removable, so you can fold up and stow ya day pack in your larger pack if we are trekking to a hut for several climbs. Or go Weilei's style and carry two packs up, but it's a bit unwieldy.
Sixth, water resistance. Could go for full waterproof pack, but that costs a premium; full dyneema or cuben fiber packs. Or can get a decently water resistant pack, Crux is a good example, can withstand anything but a full downpour. Water resistant canvas/cordura material works well. Mountain Hardwear has packs they claim are waterproof, OutDry membrane; I'm skeptical, face fabric would still wet through. Or just get a raincover, some wet spots but works well enough.
Seven, weight and durability. Can aim for something below 1.5kg. Possible to get packs under 1kg, but be mindful of durability issues. Always a tradeoff between durability and weight. Some packs use lighter fabrics on upper sections, with thicker fabrics on the bottom and other hard-wearing areas. Nice compromise that also saves weight. Problem is it introduces more seams, which water can seep through and are natural weak points that may eventual blow. Also, upper sections are delicate and can wear out prematurely. Pay attention to denier count of fabrics.
Last, but most importantly, fit. Get something that fits ya torso length obviously. Something that's comfortable, fitting snuggly against your back and doesn't swing about when you move. Where the shoulder straps connect to the main body of the bag should rest at your shoulder blades, so the straps wrap up over and around your shoulders. Shoulders and back should have a thin layer of padding. End of the day, none of the features of the pack means anything if it's just uncomfortable to wear and climb in.
Some suggestions, grouped by pricing. Insane ($300++): McHale, Alpine Luddites. High end (>$200): Cilogear 30L/30:30L/45L, Hyperlite Mountain Gear Ice Pack, Crux AK series, Arcteryx Alpha FL. Mid-range (<$200): Cold Cold World Chernobyl, Patagonia Ascensionist, Black Diamond Speed. Low end (<$100): I honestly can't think of any pack under $100 that I'd be willing to suggest. They exist, but the compromise in utility and value just isn't worth it.