I've been playing pretty much non stop since 2nd and 5th edition and early 7th (pre formation and detachment shenanigans) were my favorites from a rules standpoint. 8th is definitely fun, but it feels simplified. I like it, but I don't love it like I did the first year of 7th and pre Grey Knights 5th. The good news is that you'll pick up the rules in about 20 minutes as opposed to 2 years, and unless you try to break it there is decent balance if you play Index vs Index and Codex vs Codex. The game is faster and less convoluted, but be warned your plastic soldiers die pretty fast in 8th. Most games I've played so far are over by turn 3, because of how utterly lethal most stuff is compared to earlier iterations.
I HIGHLY recommend using the Chapter Approved method for determining who goes first, bring some psychic defense, don't skimp on your transports, and bring plenty of anti tank firepower. They've added a Command Point system, which I really like, that can be used for some nifty one shot abilities or some re-rolls. These are huge, as they can really mitigate bad luck, ensuring the player who wins played the game better and didn't lose because of a fluky run of bad dice. Also, assault is tricky. Anybody can simply walk out of a fight, leaving your dudes high and dry for the shooting phase. This was the biggest change to get used to for me. You can still stack plenty of bodies in the assault phase, but you cannot stay in unless your opponent wants to stay. The trade off is that it is now stupidly easy to get into an assault, most armies have a way to launch assaults on turn one.
Finally, there are no more templates (which I really miss...) and deep strikers can arrive whenever they want with no scatter. (But they must land 9.000001" away from all enemy models.)
TL
R - Most models move faster, vehicles are tougher, shooting is copious and reliable, the psychic phase isn't overblown but it can really make or break some armies, assaults are fast and lethal but they don't last long, cover doesn't help much, and Command Points go a long way towards ensuring a few bad dice rolls won't swing the game one way or the other.
8th feels simple and goofy at first, but it's still
40k and overall it's better than 2nd, 3rd, 4th, 6th, and the second half of 5th and 7th.