The only spoilers I bumped into were the death of Joffrey and Stannis' kid, but I'd forgotten about Joffrey by the time I'd watched it - and realistically even the the level of implausibility in Game of Thrones couldn't keep a tool like Joffrey alive for very long, so it was hardly a surprise.
Saddest bit of S4 or S5 was the death of poor 'ole Jon Snow, which was imo the most plausible 'sudden death' in the series so far. I'd also bet very good money it's not the end of him in some form, because otherwise it's odd storytelling, and while it was a shock, it's the kind of shock that kills a plot line (if it doesn't go on) rather than continues it, which is hardly what you want as the 'season ender' mid-way through a series. An odd way to do it, imo, as if they really wanted to make the return a surprise, then they should have built up to it better, and if they wanted him killed off, then tacking it on as a five-minute surprise at the end is hardly the best way.
Second silliest moment in season 4 was Oberons' death. Beyond it being telegraphed miles in advance (GoT seems to have a 'we're going to have the bad guy kill the good guy 'rhythm' now - it was a surprise in season one, and a bit in season two, but by season four it's just 'come on, we all know what's going to happen, just try not to make it too ridiculous'. I'm not sure if anyone's tried spinning from on their back to either side with pulled chest muscles, but it's not easy/sometimes not possible. Considerably harder if you've actually had someone stick a spear through your tendons. Mountain's move then was the kind of thing that I'd expect to be written by folk that hadn't done a lot of physical activity. Nice of Oberon (relatively uninjured, and very spry) to just lay there as well.
Silliest moment in S4 or S5 was Stannis suddenly turning up at the wall (noting that Castle Black is a good distance from the sea) just in time to save Jon Snow. It makes no sense whatsoever. It's unlikely Stannis knew there was a fight happening before he'd set sail (it hadn't been going long enough), it wouldn't make any sense at all to land his invasion force north of the wall if he didn't, and then you've got issues of supplying an army (with a large mounted contingent no less) on its march inland. I presume it's not quite as silly in the books and the TV series cut corners, but it was really, really odd. Also rather odd the wildlings didn't see them coming, or having tens of thousands of fighters smash 'em in the trees, but warfare has never been GoT's strong suit.
Still very enjoyable - Tyrion's journey is entertaining, and Little Finger is as chilling as ever. Varys (possibly) showing his cards and (possibly) lining up behind Danearys was also interesting. Mormont's tale was also enjoyable, but quick qn - at one stage, Mormont touches Danearys - does that mean she'll get Greyscale as well? And, of course, the plot moves on at a happy, bouncy pace to maintain interest

.
As an aside, was a little surprised Bran and co were completely left out of S5 - presuming they haven't all broken their contracts and left the show, completely dropping one of the story threads for a whole season is an odd way to do things.