spent a fair amount of time shopping for a helmet last year. This involved reading a lot and talking to as many knowledgeable people as I could. In a nutshell here's what I learned. Your experience may differ
The Liner Trick:
Manufacturers often use a thick padded liner as a cheap way to obtain comfort and fit on a wide variety of head shapes. A helmet with a thick, soft liner will feel great on your head in the store usually but.... it is also a disadvantage in the real world. 1) This is because the liner will pack down over time causing the helmet to loosen. 2) The helmet will shake and move on your head more and feel more like you are wearing it than it is an extension of your head, particularly during quick head checks or buffeting wind. 3) It may be less safe in a crash as your head will fling about more inside the shell and there is less space left under the shell for protective foam or it will require a bigger shell than otherwise.
As an example Shoei and Bell both make great helmets but if you fit one, the other will likely not fit well. Both brands are outliers on the fitment spectrum but at different ends of the scale. Most helmet models favour a particular head shape so a wide oval fitting helmet may fit great on your Asian friend but an intermediate oval may fit you better. You need to try on lots of different models and brands to determine what fits your unique head shape the best. Too tight to chew gum but not too tight to cause pain after 10 min is ideal. Shake your head from side to side. Does the helmet move with your cheeks or independently. Fitment will loosen over time as the liner packs down. Tighter helmets are safer and don't buffet or move on your head with shoulder checks. They will be less distracting on the bike.
In general precise fit tends to improve as you go up in price point and the thick liner trick is used less.
Mid to upper tier helmets also offer adjustable thickness cheek pads to get a more custom fit.
Venting and Noise:
As a general rule venting improves with price. Also some helmets are designed to block the wind and vent optimally in a race tuck while others are designed to have the rider in an upright position on a bike like the Versys. This is important as it impacts both noise and venting for the rider. Upright helmets have a different collar and chin bar position relative to the wearers head than race helmets do.
Better helmets usually have a tighter fitting collar which keeps internal noise down. Collar shape (race or touring) is important too. More venting usually means more noise too. Additional brow vents usually make a helmet cooler too.
Better (aka more expensive?) helmets have a better sealing visor. Air gaps around a closed visor are going to make a helmet noisy.
Internal sun shades are very convenient but come with a slight cost to both safety and internal venting. Safety is impacted as the internal foam must be cut away to make room for the sun shade. It also impacts the internal flow from the top and brow vents and therefore may make the helmet slightly warmer on a hot day, especially if the sun shade is up.
Helmets without an internal sun shade will require the additional purchase of a tinted visor although they are easy and quick to switch. You will also need to lug the second visor around with you in a tank bag or similar. A tinted visor is pretty much mandatory, or at least well worth the cost of entry, sun glasses, or any glasses under a helmet suck. Polarized sun glasses are superior but generate visual artifacts under most helmet shields.
If you ride in the cold or wet fogging is going to be a big issue. For occasionally riding in these conditions you can always raise you visor at a stop and use lots of anti fog but it is really not a completely effective solution. The only thing that does work is the use of a PinLock or dual pane shield. Fog city inserts are pinlock inserts that fit on shields without pins however because there are not pins to hold them securely they require adhesive and may be more likely to come off. Anti fog inserts do not last forever and may need annual or bi annual replacement.
Weight and Helmet Standards:
The weight of a helmet is dependent on the shell material, shell size and whether it is a DOT, ECE 22.05 or Snell 2015 helmet. Avoid DOT only helmets as the standard is out of date as is the research data it is based on and you get a lot more protection with the other newer standards. ECE sits in the middle, safer than DOT but not as safe as Snell. The issues you may hear about with Snell being too rigid to absorb impacts or absorb them as well as ECE helmets applied to the old Snell 2005 standard but were resolved with the replacement of the Snell 2005 standard with the Snell 2010 standard. They are the only standard to update their requirements every 5 years based on current safety research. A Snell 2015 helmet will usually be stronger and more impact absorbing than an ECE 22.05 helmet. Only Snell 2015+ helmets are allowed on a race track. All new Snell helmets for sale are Snell 2015;
Although Snell helmets are the safest in a crash they have a few disadvantages. They are most expensive usually. You can't get a sun shade in a Snell approved helmet. You can't get a movable chin bar either. They are unlikely to be the most light weight for a given size as the standard requires more impact absorbing foam, hence a larger shell size for a given head size, and they also require a stronger shell. The bigger and stronger shell makes the helmet heavier and more impacted by wind. That said no one will get a sore head or neck, even riding all day from a helmet that weighs a 1/2lb more. In fact they will only notice weight difference with comparing helmets back to back in the store.
Composite shells usually weight less than hard plastic shells and provide more protection in a crash.