The Side Mount can be a very stable position in submission grappling. It can lead to North South, the Full Mount or submissions like the Twister. In addition to all of the Twister variations there are a number of submissions from the Truck position.
One of the best arm positions is Head & Arm control. This gives you a lot of control over your opponent's upper body.
Other arm positions like both-arms-past or both-arms-near-side are not as effective without the gi. These are great arm positions in BJJ.
Each opens up different options for you. But in no gi they both allow your opponent too much movement. The result is that they can put you into Half or Full Guard. Even worse, they could get back to their feet which is no good in MMA if they are a better striker than you are.
Judo side control is another solid position from the side. There are also a number of submissions from Judo Side Control. But there are also a number of escapes.
The Twister Side Control is a way of playing the SM that Eddie Bravo developed. He used it primarily to setup the Twister in competition. It is also a great way to gain the Full Mount or the North South position.
The knee-on-stomach is another option from side control. It is not as effective without a gi but with solid Head & Arm control you can still work it. I personally do not use knee-on-stomach much in my game. Even when it is tight I feel like there is too much space.