I was on lithium, and for me it did not cause any major side-effects. Only a slight tremor and mild nystagmus (only occasionally, and this one is supposed to be very rare). I didn't have any weight gain. In general, as psych meds go, lithium is reasonably mild on side-effects. I must add that for me, it didn't really help much, either. Reducing manic symptoms, maybe, but not depression. It seems to work great for many people, though.

Of course there's always the risk of long-term complications, but they monitor for that. I've heard that kidney damage by lithium is mainly caused by repeated occasions where the lithium levels in your blood become toxic. The toxic levels are relatively close to the therapeutic levels. So the thing is to take the minimum therapeutic dosage that works for you and make sure you drink well. If you do that and have occasional check-ups, you should be okay.

When I first started taking psych meds I was also terrified by the huge lists of side-effects. Now I only look at the "very common" and "common" side-effects (as they are often listed) and I don't even consider the rest because the chance that I'll get them is very small. I'll cross that bridge when I come to it. If there's nothing in the most common side-effects that's an absolute no-go for you, then I wouldn't worry too much. You can always stop if it doesn't work for you.