I'll go with the 4 GBs of RAM (3 GBs for games) the Switch has. Originally, Nintendo was just going to go with 2 GBs total like WiiU had (1 GB for games) until Capcom's input managed to change the company's mind. I can just see Nintendo pulling out its calculator, typing feverishly to see just how much more it would cost before begrudgingly agreeing to it in the end.

Pretty sure that decision helped immensely in contributing to the excellent 3rd party support the Switch received.

There are several surprises, like booting it for the first time with BoTW card inserted, which was amazing, magical even, but if I had to choose just one, it would be playing DOOM (2016) on a tiny ass chipset+screen and having a blast in the beginning of the console's life cycle. People would envy me on airplane flights seeing that beautiful game running on the palm of my hands back then. It is far from being the game I played the most (I guess it is Rise+SB, then SSBU, followed by the 2 Zeldas, and, recently, F-Zero 99), but it was not something I had considered after purchasing the system, so it is probably a great surprise right there.

Games that pleasantly surprised me are Final Fantasy 7 and 8, Suikoden 1 and 2, and Witcher 3.
Many people are saying Xenoblade and Zelda. Maybe it's just me, but those were pretty obvious games that would be on Switch. Or at least, not particularly surprising.

Ok, the other day I made a comment about YouTubers not leaning into retrospective Switch videos as we wait for the Switch 2. This is exactly the kind of video they could be making instead of chasing worthless leaks every day.

Anyway, biggest surprise of the Switch?

Probably Breath of the Wild and how massive it was running on a handheld. Also, the way that that single game basically Phoenix Downed Nintendo from the depths of their struggles with the Wii U.

By the same token, I would say games like No Man’s Sky and Subnautica for the sheer size and scope of their worlds being brought to life on the system was pretty amazing and have been some of my favorite experiences on the Switch.

Oh, and then there was Labo and Ring Fit.

Labo failed, but the buzz around the reveal was palpable. I found so many great YouTube channels watching everyone’s reaction to the first video. While it may not have been as fun as it looked or whether people didn’t think it would be as fun as it looked, it was still great to see Nintendo leaning into their quirky side once again.

And then there’s Ring Fit. Quirky, wild, positive, and a sales beast. Who knew Nintendo basically had another WiiFit up their sleeves. I’m still wondering if we’ll get a sequel or even a line of games that use the ring peripheral.

The hybrid feature.

Nintendo had always released 2 consoles; home and handheld. They easily dominated handheld while their home consoles lacked significantly.

Focused on hybrid was their smartest move. It significantly reduced overall cost by producing only one console while able to cater to both; the home and handheld gamer.

Switch deserves all the success it receives.

Pokémon Legends Arceus. I was on the fence about buying it at all but ended up having the most amazing time with it and explored every nook and cranny.

Astral Chain was really cool too.

Breath of the Wild - I wasn't surprised this would be on Switch at release, but its reveal at E3 2016 was a huge surprise, and it was cool that Nintendo let it be the focus of their entire E3.  After several years of the failed Wii U, they revealed this amazing Zelda game.  And after watching hours of E3 footage, I knew the Switch would be successful and I would get to play a Zelda game I'd enjoy as much as the NES/SNES days.  I literally waited 25 years for this game to be released.

Honorable mention goes to Hyrule Warriors: Age of Calamity and Cadence of Hyrule.  Both are great games.

I think the Tomb Raider remasters and Powerslave Exhumed. Never thought I'd be playing them on the Switch.

Metroid Dread was a big one for me on multiple fronts.
On the one hand, "Dread" was legendary vaporware that I never thought would see the light of day, but also I was skeptical of how good it would be since I didn't much like Mercury Steam's previous games.
The finished product though blew me away, and is one of the finest games of recent years in my opinion.

Ring Fit Adventure - Made moving my body actually pretty fun.