There are never really hordes of tourists but insects can be a problem. The days are long and the locals make the most of this with life being very much outdoor orientated. Book well in advance for tickets at this time. Autumn is a beautiful time, especially if you can catch all the colours around the end of September. The weather is great during the day but at night the first signs of winter can be felt as the thermometer plunges.
Winter is the least visited season but, many argue, the most beautiful.
Siberia actually looks like people imagine it with the landscape blanketed in snow and by the New Year Baikal and most rivers are frozen solid. However, the locals know how to keep warm and everything from the inside of gers to the train cabins are well heated. The main factor is likely to be your level of consumption of alcohol and souvenirs. Moscow and St Petersburg, where you will only have accommodation with breakfast paid for in advance, are more expensive. In both these cities transport on the metro is cheap but the cost of getting into the sights as well as eating out aren't far off European and North American levels.