What would happen if the earth were not tilted?

There would be no seasons. The seasons are a result of the sun's light reaching the Earth at a tilted angle. (Mars also has seasons.)

No Seasons, and Tides would be influenced to some degree.

http://www.enchantedlearning.com/subjects/astronomy/planets/earth/Seasons.shtml

This was something I had never really given any thought to until today. I'm sure somewhere in my schooling I was taught the answer but with many things, if ya don't use it, you lose it! So I looked it up and this is what I found:

We must be assume that this change in the earth's orbital axis is a thought experiment. The earth can be thought of as a gigantic gyroscope, spinning at over 1000 miles per hour at its equator. If its axis were to be suddenly shifted from its normal inclination, inertial forces produced in such a change would literally tear the earth apart.
That aside, if the rotational axis of the earth were perpendicular to the plane of the ecliptic (the plane that passes through the earth's orbit), several dramatic differences would appear. First would be that the length of day and night would be exactly equal, over the whole planet. There would be no seasons, just one average of the current four. Weather patterns would be very different; much more uniform.

These and other changes in the oceans and atmosphere would result in tremendous and unpredictable changes in conditions on the earth's surface including shifts in the oceans' levels and changes in the location and extent of arid as well as tropical areas.
Answered by: Scott Wilber, President, ComScire - Quantum World Corporation
http://www.physlink.com/Education/AskExperts/ae495.cfm

"The sun would always be just on the horizon 24 hours a day on every day at the poles. Every day would be like what it currently is on the equinox since every location on Earth would have about a 12 hour sunlight days and the noon sun angle would be about the same every day. There would no longer be season as we know them. The temperature and precipitation pattern would not vary much. It would still be warm at the equator and cold at the poles. The most profound impact on temperatures would be at the poles. Instead and dark and frigid temperatures in the winter and warmer and constant sun in the summer, the polar areas would have much more uniform temperatures year round and the sun would always be low on the horizon. Across the Earth it would be like it is in the middle of fall or spring but it would last all year every year. Areas today that have wet, dry, warm and cold seasons would have a fairly constant weather all year whether it be wet, dry, warm and/or cold."


http://www.theweatherprediction.com/habyhints2/471/