I learned this in school

KISS= Keep it simple stupid. 

The more you keep games simple to control but a challenge to play it helps drive people to want more. Take Super Mario for instance. People love simple platformer mechanics but a challenge.

No real buttons.

If I were the developers I would rather create a game for handhelds. Here's my complaints

1) Smartphones do not have buttons or joypad so the control by just touching screen will be difficult for me.

2) I have Whatsapp, Viber, Facebook, Twitter, etc......so everytime when I play the game there's always a new notifications pop up from these social apps which disturb my gaming session. Sometimes the game is paused by itself and sometimes it crashed, which sucks btw

3) personally I need my phone wherever I go. I need to make a call, messaging, browsing internet, recording my lectures and sometimes listening to mp3, so I need a long-span battery to do any other things besides gaming.

For me in a large part is the lack of depth, many utilize the concept of F2P and making games that are suited to only be played for short bursts of time, which makes sense for the platform and the people that are likely to play the games. Making an in depth game might not work too well because the platform and users aren't really suited for that. I don't mean to discourage though, there is a chance it could appeal to the smaller demographic that do enjoy the kind of gaming you are working towards. I'm not one personally so that's likely why I view it like this. I would prefer the game just to be released as an indie entry on one of the other systems. 

Along with that the lack of good controls. 

ads, freemium system.

Gameplay is the worst thing. Touch pad is shit, inherently unresponsive and inaccurate, so not adapted at all for gaming.

I'd think 8bit / 16bit era games could have a second life on mobile platform because of their simpler controls.

I think turn based strategy games could be viable on mobile platform. Gimme the possibility to replay all Fire Emblem and I'd throw my money on my phone.

They are limited in every technical aspect possible and either want to flood you with adverts or sell you microtransactions.

Hardly any come with a story line or context and the ones which do are the cheesy ones trying to be funny but failing to realise a hundred thousands devs have beaten them to it and the end user isnt amused anymore.

Theres plenty of emulators available... but it wont be as fun as it is in theory.

The general nature of mobile games is what annoys me.. Most of them are either infected with microtransactions, are time-wasters that become boring quickly or simply have uncomfortable controls!

The kind of games that I like can't be played on them.