Noobie Questions (No Spoilers pls!)

ME1 does allow the player to play the side missions right up to the point of the ending sequence... once unlocked, none will disappear.  The game will frequently hint at time sensitivities, but I can only think of (right now at least) only one short timed sequence in the entire game where a mission failure will result if you don't beat the clock... and you will simply have to reload at that point since you have to pass that mission to finish the game.

These are the main missions you're given once you leave the Citadel:  Feros, Noveria, and the planet in the Artemis Tau cluster.  You will be clearly given another by the Council once you do 2 of those 3 and you can do whatever 2 those are (i.e. the one you have remaining and the new one) in either order.  Once you do all 4, you will be forced into returning to the Citadel and the game will start you into the ending sequence.  Although it's not great story-wise, however, you can still do any remaining side quests after leaving the Citadel on that occasion as long as you do not go to the last mission planet. 

I would, however, recommend that you save one of the previous 4 main mission planets until after you have completed all the side mission planets that you wish to do (it just works better story-wise).  If you don't want to do all of them, I would prioritize the two that two of your alien squad mates will give you coordinates for once you talk to them and a group of planets in the... (spoiler contains name of cluster) which is actually associated with your third alien squad mate (although it's not critical to ME2 if you miss it).


I would also ensure that you collect enough items to clear those collection quests in your journal (although only 1 of them has an impact in ME3) - unless you want to me to tell you which one of those collection quests it is so you'll be able to skip over the others.    You'll also want to make sure that you finish all the encounters on the Citadel as many of these result in little events in the subsequent two games (this means you should return to the Citadel voluntarily at least a couple of times before doing all of the 4 main mission planets mentioned in my first paragraph.  These include a couple that will send you off to other planets to complete.  There is also one side mission Hackett gives you over the comms when you reach level 20 that you'll want to be sure to do as this unlocks your Specialization Skill.