Jamming

I'm just getting into playing solos over some chord progressions, and I've run into a wall. Most of the stuff I've learned on my own is luck- but there's this song that goes G, D, Em, C. Since it's in the key of G, I thought I could play the G major scale. Well, it sounded like crap. But when I played Em, any note I hit sounded good. Why? Is there anyway to find scales that are composed of notes that are 'safe' to play in any key? I'm aware that the major scale is one continous pattern and the natural minor is extracted from the major, but where do you start when playing? Is there any completely 'safe' scale, or do you have to change when the chords change? I've only been playing for a year, and all the guitarists I know play by ear, so they're clueless. Could someone give me a start?
Thanks.

I don't know of any scale that would have all the correct notes in any key except the chromatic scale but you have to know your music theory pretty well to figure it out. Also in the major scale the patterns are major, minor & diminished just like a key:
Key:G Major
Ionian - Major - 1 chord - G Major
Dorian - Minor - 2 chord - A Minor
Phrygian - Minor - 3 chord - B Minor
Lydian - Major - 4 chord - C Major
Mixolydian - Major - 5 chord - D Major
Aeolian - Minor - 6 chord - E Minor
Locrian - Diminished - 7 chord - F# Minor or Diminished
You should start on the note G if you're starting with a G chord, that might help. But don't worry about it too much you've only been playing for a year, learning how to solo well takes years of time and practice.

G Major and E Minor contain exactly the same notes (G, A, B, C, D, E, F#), but the difference is in the way that the PLAYER emphasizes these notes. You say that when you played in E Minor it sounded good, but it sounded bad in G Major. Well, the reason is the way that you approach and play the minor scale as compared to the major scale. Both should work fine. The safest scale you could use would be the G Major Pentatonic (or E Minor Pentatonic scale where once again, these have exactly the same notes). This way you avoid heavy tension notes like the 4th (C) adn the 7th (F#). Try it out and tell me how it goes (although I know it should go well, because I've soloed over that progression many a time with old G Major Pent...)