Currently there is indeed no monetary incentive to run a full node. It's done by enthusiasts that just do it for the fun of it, or by projects who are building on top of Ethereum and need access to network data (also wallets run it at the moment because light clients aren't done yet, they will certainly switch away s soon as they can).
The inherent problem with light nodes is that they leach the network: why run a full node when you can get away with a light node? To address this we're considering a model where light nodes actually pay for data they request from full nodes. If we can work out a meaningful model for this, then this would be the incentive for people to run full nodes: they can serve light nodes, and be compensated for it.
Also we're working on Swarm, which would allow anyone to rent out their disk capacity for others to use as a distributed file system, which could also net full nodes some extra income for their efforts.
Q: I know that when a new geth instance runs it connects to the bootstrap nodes. What I want to know that do other nodes download blockchain from the bootstrap nodes or nor?
A: No. Regular nodes do not download the blockchain from bootnodes. The only role for the bootnodes is to allow regular nodes to connect to and get the connection information on other regular nodes. The regular nodes will then use this information to connect to other regular nodes.
Q: Bootstrap nodes are highly available so why just simply connect to bootstrap nodes instead of connecting to some other node in the network.
A: The bootnode's only role is to provide the connection information for regular nodes to connect to each other. The connection information for several bootnodes are hard-coded into the regular node source code - see bootnodes.go
As the bootnodes only serve very little information to each regular node, the bootnode server resources can be focussed on providing this information and not get bogged down transferring the blockchain data.
Q What if bootstrap nodes provide wrong information. The whole network is compromised right?
A Not necessarily. You are able to specify new bootnodes on the command line parameters of the regular node client:
geth help | grep bootnode
--bootnodes value Comma separated enode URLs for P2P discovery bootstrap
You are also able to manually enter a list of other regular nodes on the network using the admin.addPeer(...) command.
Recently many users reported that their regular nodes was not able to find peers. There were some problems with the bootnodes. See geth does not sync out of the box.
The temporary workaround was to get a list of regular node connection information from peers that were already connected to peers, and pasting this connection information into the regular node that was experiencing the peer discovery problem. See How can I create a list of peers from my syncing geth node to manually paste into my non-syncing geth node using the admin.addPeer() command?.
Q: How is a regular node different than a bootstrap node?
A The regular node has the peer discovery mechanism of the bootstrap node. Additionally, the regular node will download and synchronise the blockchain with other regular nodes. The bootstrap node cannot be used directly to synchronise the blockchain.
Q: And also How does peer discovery actually works?
A: See Node discovery protocol and What are the peer discovery mechanisms involved in Ethereum?
Q: What if nodes only download and not upload?
A: There are sufficient nodes (~6,000) on the network that your node should find other nodes to download from.
Q: What if I execute a denial of service attack which connect to many nodes in the network and downloads blockchain?
A: You will need a lot of computing and networking resources to try to execute a denial of service attack on ~6,000 nodes.
Best Answer
You don't need to buy or register an Ethereum node. You can simply download one of the clients such as go-ethereum and run it on a computer connected to the network.
Ethereum nodes will automatically connect to other nodes and synchronize a copy of the Ethereum blockchain, so there's no formal registration needed.
If you want to stand up a mining node, that will require some advanced setup. Either way, I'd highly recommend you read through Ethereum's documentation before you get started.