Having been a regional pilot up until 2001 I can tell you in no particular order:

Better planes that can get to altitude and go fast when they are there, long layovers in places other than DAY, CID, FNT, LEX, etc., much better schedules (reserve typically gives around 15-18 days off a month), no high speeds/continuous duty/naps (redeyes replace that but they are typically preceeded by a 24 hour layover), better retirement, jetways everywhere you go, overhead bin space for pax, 2nd year pay that exceeds 18 year pay at the regional you left, etc. Also, there is something cool about Boeing on the type rating section of your certificate. But this is just for starters. Others may chime in perhaps.