LID - A lid keeps the rain out of the main compartment and it usually houses at least two pockets so you can keep your gear organised. Lids add weight to the pack though and very lightweight packs often don't have a lid as such, they make do with a weather resistant closure of some sort, keeping the weight of the pack down means you have to carry less, making your trek easier. Some packs have a removable lid so the choice of whether to take it or leave it is up to you.
SHOULDER STRAPS - A backpacking rucksack will aim to transfer the load to the hips so the shoulder straps in this case are mainly for stability - if a rucksack doesn't have a decent hipbelt all the weight will be taken on the shoulder straps and so they need to be well padded.
LUMBAR PAD - Usually found on packs designed for moderate loads and above - the lumbar pad helps the pack transfer the load comfortably to the hips.
LOAD TENSIONER STRAPS - These join the top of the pack with the top of the shoulder straps and by pulling them tight the pack is pulled closer into your back - this in turn moves the load closer to your centre of gravity which makes it easier to carry.
BACK PANEL - The back panel separates the load from your back - on very lightweight packs this may be just a sheet of fabric, an upgrade from this may be a foam sheet to stop the contents of the pack digging into your back. Packs designed for heavier loads will have a frame of some sort - this can be a stiffened foam pad or a metal frame which supports the load much better. Its the internal frame that transfers the weight of the pack to the hipbelt and pack designers will spend a lot of time trying to get this right. Finally there are back panels with venting - some are designed to create a void between your back and the rucksack - these are great at venting your back but move the load away from your centre of gravity making it seem heavier. Other venting options involve holes or channels cut into the back panel to allow air to circulate - typically with only mixed success.
CHEST STRAP - A chest strap keeps the shoulder straps from drifting apart, a worst case is that they slip off your shoulders, but typically loose shoulder straps pull down and back on your shoulders making the rucksack uncomfortable. A few people don't like chest straps because they can press on the chest, however they can be loosened easily when not required. Look out for some mechanism whereby the attachment point can be varied up and down the shoulder straps - this is essential to accommodate different users.
SIDE COMPRESSION STRAPS - A 60 litre rucksack with only 30 litres in it can be unstable, and the load settles to the bottom making it seem heavier. Most large packs have 'compression straps' which reduce the volume of the pack making it easier to keep the load stable and near to your centre of gravity. On long backpacking trips your load volume could change considerably depending on how many days food you are carrying.
TREKKING POLE TOTE - Some rucksacks have specific attachment methods for trekking poles that make stowing these when you are not using them much easier and more secure.
SIDE MESH POCKETS - more or less essential these days - side pockets can take an amazing amount of gear - load them up with things you'll need on the trail such as water bottle, waterproofs, water filter etc.. using these saves you opening your pack up everytime you need something.
FRONT POCKET - often constructed from stretch mesh a large pocket on the front is ideal for a wet flysheet plus a host of other items you might want to put in there.
HIP BELT POCKETS - these are the best invention ever - keep your trail food, compass, suncream etc very handy, accessible without taking off your pack or dislocating your shoulder trying to reach back to the side pockets. They come in various guises and sizes, most are zipped and stretch mesh.
LOWER COMPARTMENT - two compartments can help keep you organised or separate wet gear from dry, however they generally add significant weight to a pack and some people don't think its worth it.
RAIN COVER - some packs have an integrated rain cover. A rain cover is only worth carrying if you expect a lot of rain - they do not make the pack waterproof because they don't cover the back panel - their main function is to stop the fabric of the pack soaking up water and becoming heavier as a result - this will only happen if there's a lot of rain for a long period of time - therefore its only worth carrying the extra weight of a pack cover if you expect this kind of weather for most of the days you are trekking.