Spiders (EDH / Commander) [Révision 61]

This is not a good deck. It's mono-green spider tribal with a Fog subtheme. Many of the included cards are included for flavor; namely Hidden Spider, Witch's Web, and even the commander, Arasta of the Endless Web. There are many better mono-green tribes you can build, Spiders as a tribe do better in Golgari or Jund colors (not that Spiders are a particularly good tribe even in better colors), and overall better tribal support exists.
It was fun to build, and it's rather fun to play, and that's all I intended with the deck, so by those standards, it's a complete success.
The primary issues with the deck are inconsistent card draw, lack of tribal synergies, very little removal, and no clear win-cons.

If you want to improve it, you can:

A. Choose a new commander. Ishkanah, Grafwidow and Shelob, Child of Ungoliant both seem to be very good commanders and give access to black, which gives you access to much better card draw, removal, and some more efficient creatures. Ishkanah is a good graveyard-lifedrain focused deck, while Shelob is an actual spider-tribal commander. Thantis, the Warweaver would give access to red as well as black, though adding red doesn't really help the spider theme much and Thantis doesn't do anything to help spiders either. Still an option though.

B. Remove the tribal focus. As I said, Spiders aren't a particularly good tribe (even with the LotR cards, there's not much to go with), having no specific tribal support except for one Arena exclusive card and now Shelob, Child of Ungoliant (unless you want to count Arachnogenesis as tribal support). Without being a spider tribal deck I don't know why you would continue to use Arasta as your commander; she's not that great as a commander (good card for the 99 though).

C. Be willing to spend more money. If you want to keep it mono-green spider tribal, cards like Vanquisher's Banner, Herald's Horn, Kindred Summons, and Cavern of Souls would help the deck. Sylvan Library, Craterhoof Behemoth, and Doubling Season would also help a lot, but are stinking expensive. I try to build budget (with a few cards as exceptions because I already own them (lucky me because the few good spider cards have gone up in price since the LotR cards were printed)) so none of those cards are included, despite how good they may be and how well they fit in the deck.

D. Remove flavor/change or remove the subtheme. A lot of cards are included because they featured a spider in some way as I previously mentioned. They don't help with the synergy of the deck, but they look nice. The only reason there's a Fog subtheme to this deck is because of Terrifying Presence. That's not to say that the Fog subtheme is completely bad, as it does help to compensate the lack of removal and boardwipes in mono-green, but there's probably better cards or subthemes to make use of (the actual viability of Fog effects is a discussion for a different day).

E. This kind of goes in with my first point, but you could also make it a Selesnya toughness matters deck. There are two selesnya spiders you can use in Ancient Spider and Pale Recluse if you really want to keep the spider theme (not that they're particularly good creatures though). This would require require finding a new commander; Trostani, Selesnya's Voice would work well. Making it Selesnya would also give access to more card draw and removal.