Sequencer: the single computer that all your L2 traffic passes through

The sequencer is a node that accepts L2 transactions, orders them and packs them into batches for Ethereum. The vast majority of rollups have just one, and it's owned by the team. All the network's "decentralized" activity passes through a single server.

What this means in practice

• Censorship: the sequencer can choose not to include your transactions. An escape hatch via L1 exists, but it's slow and expensive.
• Downtime: if the sequencer goes down, the entire network stops. Major L2s have gone offline for hours more than once, and the market kept moving without you.
• MEV and ordering: whoever orders transactions decides whose transaction comes first. That's power and money.

Why they aren't decentralized

The sequencer is a revenue source: the spread between user fees and the cost of publishing to Ethereum stays with the operator. Teams aren't in a hurry to hand that cash flow over to external validators - sequencer decentralization roadmaps have been stuck in "coming soon" status for years.

The takeaway for users is plain: L2 is convenient and cheap, but in terms of trust model it's closer to a "fast database with an exit guarantee" than to Ethereum. L2 is a place to keep working capital, not savings.