About 30 missions. Skill building and weapon mods add to replayability. SP isn't that great cause the AI doesn't help all that much for mission objectives, but coop is FUN. (Biased fan of pd1, but have played pc beta)
The levels are small, not open world. You open crime.net and choose a heist that is available to you. Every time you play a level, it's different due to random elements (random placement of items, assets if you purchased them, difficulty level, etc.) so there's the replayability factor. Also, having four different classes to spend skill points in adds to this.
Like shaakes said, playing online with people is the way this game is meant to be played.
Payday (at least the first one) is not near as much fun if you play alone. Besides the obvious advantage of having a human instead of AI, you'll have access to fewer supplies (bots don't carries ammo or health bags) and will have too personally do all objectives. Honestly, if you don't have Internet you should give it a miss, since the frustration will outweigh the fun. However, game kicks a** with other players (at least the first one does.)
I have intertent. just no friends online to play with. so I will get it and play solo.
OK- just remember what we told you about single player... seriously. The stuff worth doing is not worth doing solo. It can be played solo but it's not really a SP game. Hard to really explain until you get some hands on time with the game. And if you have internet, but no friends to play with you'll be able to jump into a few random games and find some people you like to play with.
I'm getting it for sp only, too. I enjoyed the first Payday very much playing single player only. I very much dislike playing with other people. I got into video games when I was a kid because it was something I could do alone. So multiplayer games defeat the purpose for me. I enjoy Payday just fine by myself; I'm sure you can too.