I finished ME3 last night and thought it was an ok game but I feel like I must have missed alot of content.

I did everything that was available but I ended the game at level 41, looking at the achievements it's possible to hit level 60 somehow; did I really just play 2/3rds of the content, because that is how it felt.

Towards the end of the game, there were quite a few points on the galaxy map that I couldn't travel to and had never been able to travel to, is that normal?

Does multiplayer have any effect on the single player game content wise? I was able to fill my green bar completely without doing any multiplayer, but my readiness rating stayed at 50%, that didn't seem to matter at all.

For the leveling it depends if you imported a save from ME2, as it starts you off at the level you were in that save, so for people who were at level 30 already it was possible (but not quite) to get near to 60 in one playthrough. So if you started fresh with a level 1 dude, being 41 at the end seems totally normal.
 
Yes that is normal, some points never open up, others get closed off as the game progresses.
 
Only effect is if you want to get the extra few seconds cutscene at the end, but it's easier to just look it up on youtube if you don't want to bother with MP. But content wise - no.

Did you do all the "Hey here's that ME2 character" missions, the N7 missions and the side missions on Tuckanka and Rannock. If so then your good I think. The main thing I can think is the extra DLC experience from ME2 and ME3 you may have not earned, assuming you didn't buy them. That level does seem kinda low however; I finished at about 57.

Unavailable points on the map are normal.

I didn't import from ME2 because I switched platform, but I did everything that was available to me including the DLC for ME3, I never saw Kasumi or Jack, really the only ME2 character I saw was Jacob but I'm guessing that's because I didn't import.

Thanks guys.

You start at lvl 30 if you import a max lvl ME2 character, making it fairly easy to hit lvl 60. Also, there are certain relays on the galaxy map that don't have any explorable systems.

I think it's tweaked so that you need to import a completed ME3 game and play on insanity if you'd like to hit 60. That, or wait until enough DLC is available so you have enough single-player content to do it in one playthrough. Alternately you can just promote a lvl 20 multiplayer guy.