1. Research-y projects. Big companies have the resources to place long-term high-risk bets on research projects that will probably fail. Some startups have a single such high-risk bet, but you can't just decide to work on whatever crazy idea you come up with because the company can't really absorb your salary for years if the bet doesn't pay off.
2. Really broad scope. One cool thing about working at a startup was that I could do whatever I wanted as long it was relevant to the startup. In fact, that was the biggest reason I went with a startup just out of school; I would have been some tiny cog on a giant team at a big company (that's not inevitable, but that's what the offers I personally had entailed). But I now have enough credibility that I can go off and do whatever I want at a big company. Since the big company has a much larger scope than a startup, there are many more things I can work on.
3. Resources. At the startup, I could take 100 machines to run an experiment and it wasn't a big deal. If I wanted to use 1000 machines I'd have to get people to ok it because I'd be eat into resources that were necessary for the company's operation. Now, if there's something I'm curious about, I can run a map reduce across the entire internet.
4. Relaxed environment. The vast majority of my team is married with kids, so the office is deserted by 5pm. There are a couple of folks I've seen stay late for a week or two to hit some deadline, but that's like one or two weeks out of the year. The startup was one of the most relaxed startups I've ever seen, but there were still month long crunches of 50+ hours/wk as some emergency came up. In principle, a startup could be as relaxed as a big company, but that's extraordinarily rare. At the big company, one of the most respected people in the office casually remarked to me that he's been at negative available vacation for all but his first two weeks at the company (and he's been here for seven years). At the startup, the most respected people all lost vacation every year because pushed up against the vacation cap.
5. This should really be like 25 or something, since it's not in the same league as the others, but I'm making a lot more money than I was at the startup.
Don't get me wrong -- there are a lot of advantages to being at a startup, and I'm glad I spent a good chunk of my career at one. I'm just saying that there's a lot I'd have to give up to go back to a startup, and the things on that list don't even make the top 30.