First things first: Check with the seller to see if it's the shorter nano base or the longer XPR base. Both types of batteries will work in any 18 volt Dewalt tool, but the shorter nano batteries might look funny if the saw has a longer base, and the old XPR batteries might look funny on the nano base.
1) 4 blade positions (up, down, left, right)
2) Lock button (black button an inch northeast of the trigger, see picture). This enables you to carry/store the saw with the battery in place without the ability to accidentally pull the trigger and saw something you didn't mean to (your leg, etc).
3) Removable shoe. The round yellow button towards the front allows you to remove the "shoe" (Black metal piece at end of saw).
4) The tool's not that heavy, I cut a high branch off a tree holding the saw with my arm almost vertical without any problem.
Corded vs cordless: I've had a corded sawzall for about ten years. Worked great when I finished my sister's unfinished basement. I used it over and over and over again. If you just want one for demolition work, or any indoor work where there's an outlet, you might want the corded, especially if you'll be using it a lot continuously (won't need to keep recharging batteries).
That said, I've had branches in the yard for years that I never got around to dragging out the 50 or 100 foot extension to use with the corded sawzall. With this cordless Dewalt that I got just a few weeks ago, I've been filling four yard waste cans a week with fallen branches and dead limbs cut with this saw. And no longer will I put 5 foot plus branches out hoping that they take them, just cut them with this saw. It's actually fun (for now at least).
So for a summary of corded vs cordless: If you have a lot of indoor work, maybe get the corded, you won't have to change batteries (and corded are generally more powerful). If you just use it a few minutes at a time, or have a yard with large branches to cut, cordless is the way to go.
For BIG branches, you can get 12 inch blades here: DeWalt DW4804 12-Inch 6 TPI Taper Back Bi-Metal Reciprocating Saw Blade, 5-Pack
Update: As long as the branches aren't too thick,the Black & Decker NPP2018 18-Volt Cordless Electric Pole Chain Saw, 8-Inch Bar is much better at cutting branches. It's faster, quieter, and doesn't vibrate like the sawzalls, and you can reach much higher without a ladder. I doubt you could cut a 7.5 inch diameter branch with it sideways though (I did with the Dewalt "sawzall"). You could try cutting such a branch from a tree straight across, just make sure someone's filming so you can make 600 bucks on one of those video sites when the branch falls the wrong way and destroys your shed/porch/car/etc.